Carbon matrix- and carbon matrix composite-based dendrite-intercepting layer for alkali metal secondary battery

ABSTRACT

A dendrite penetration-resistant layer for a rechargeable alkali metal battery, comprising an amorphous carbon or polymeric carbon matrix, an optional carbon or graphite reinforcement phase dispersed in this matrix, and a lithium- or sodium-containing species that are chemically bonded to the matrix and/or the optional carbon or graphite reinforcement phase to form an integral layer that prevents dendrite penetration through this integral layer in the alkali metal battery, wherein the lithium- or sodium-containing species is selected from Li2CO3, Li2O, Li2C2O4, LiOH, LiX, ROCO2Li, HCOLi, ROLi, (ROCO2Li)2, (CH2OCO2Li)2, Li2S, LixSOy, Na2CO3, Na2O, Na2C2O4, NaOH, NaX, ROCO2Na, HCONa, RONa, (ROCO2Na)2, (CH2OCO2Na)2, Na2S, NaxSOy, or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbon group, x=0−1, y=1−4; and wherein the lithium- or sodium-containing species is derived from an electrochemical decomposition reaction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. patent application Ser.No. 14/545,552, filed on May 21, 2015, (now U.S. Pat. No. 9,780,349),which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention provides a dendrite-intercepting layer for arechargeable lithium metal battery (having lithium metal as the anodeactive material) or a rechargeable sodium metal battery (having sodiummetal as the anode active material, such as the room temperature Na—Scell that operates at a temperature no higher than 100° C.).

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (e.g.Li-sulfur, Li metal-air, and lithium-metal oxide batteries) areconsidered promising power sources for electric vehicle (EV), hybridelectric vehicle (HEV), and portable electronic devices, such as lap-topcomputers and mobile phones. Lithium as a metal element has the highestcapacity (3,861 mAh/g) compared to any other metal. Hence, in general,Li metal batteries have a significantly higher energy density thanlithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds, such as TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, asthe cathode active materials, coupled with a lithium metal anode. Whenthe battery was discharged, lithium ions were transferred from thelithium metal anode through the electrolyte to the cathode, and thecathode became lithiated. Unfortunately, upon repeatedcharges/discharges, the lithium metal resulted in the formation ofdendrites at the anode that ultimately grew to penetrate through theseparator, causing internal shorting and explosion. As a result of aseries of accidents associated with this problem, the production ofthese types of secondary batteries was stopped in the early 1990's.

To overcome these safety issues, several alternative approaches wereproposed in which either the electrolyte or the anode was modified. Thefirst approach involves replacing Li metal by graphite (a Li insertionmaterial) as the anode. The operation of such a battery involvesshuttling Li ions between two Li insertion compounds at the anode andthe cathode, respectively; hence, the name “Li-ion battery.” Presumablybecause of the presence of Li in its ionic rather than metallic state,Li-ion batteries are inherently safer than Li-metal batteries. Thesecond approach entails replacing the liquid electrolyte by a drypolymer electrolyte, leading to the Li solid polymer electrolyte(Li-SPE) batteries. However, Li-SPE has seen very limited applicationssince it typically requires an operating temperature of up to 80° C. Thethird approach involves the use of a solid electrolyte that ispresumably resistant to dendrite penetration, but the solid electrolytenormally exhibits excessively low lithium-ion conductivity at roomtemperature. Alternative to this solid electrolyte approach is the useof a rigid solid protective layer between the anode active materiallayer and the separator layer to stop dendrite penetration, but thistypically ceramic material-based layer also has a low ion conductivityand is difficult and expensive to make and to implement in a lithiummetal battery. Furthermore, the implementation of such a rigid andbrittle layer is incompatible with the current lithium batterymanufacturing process and equipment.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of graphite anode is <372 mAh/g and that of lithiumtransition-metal oxide or phosphate based cathode active material istypically in the range of 140-200 mAh/g. As a result, the specificenergy of commercially available Li-ion cells is typically in the rangeof 120-220 Wh/kg, most typically 150-180 Wh/kg. These specific energyvalues are two to three times lower than what would be required forbattery-powered electric vehicles to be widely accepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. Amongvarious advanced energy storage devices, alkali metal batteries,including Li-air (or Li—O₂), Na-air (or Na—O₂), Li—S, and Na—Sbatteries, are especially attractive due to their high specificenergies.

The Li—O₂ battery is possibly the highest energy density electrochemicalcell that can be configured today. The Li—O₂ cell has a theoretic energydensity of 5.2 kWh/kg when oxygen mass is accounted for. A wellconfigured Li—O₂ battery can achieve an energy density of 3,000 Wh/kg,15-20 times greater than those of Li-ion batteries. However, currentLi—O₂ batteries still suffer from poor energy efficiency, poor cycleefficiency, and dendrite formation and penetration issues.

One of the most promising energy storage devices is the lithium-sulfur(Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and thatof S is 1,675 mAh/g. In its simplest form, a Li—S cell consists ofelemental sulfur as the positive electrode and lithium as the negativeelectrode. The lithium-sulfur cell operates with a redox couple,described by the reaction S₈+16Li⇄8Li₂S that lies near 2.2 V withrespect to Li⁺/Li⁰. This electrochemical potential is approximately ⅔ ofthat exhibited by conventional positive electrodes (e.g. LiMnO₄).However, this shortcoming is offset by the very high theoreticalcapacities of both Li and S. Thus, compared with conventionalintercalation-based Li-ion batteries, Li—S cells have the opportunity toprovide a significantly higher energy density (a product of capacity andvoltage). Assuming complete reaction to Li₂S, energy densities valuescan approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on thecombined Li and S weights or volumes. If based on the total cell weightor volume, the energy densities can reach approximately 1,000 Wh/kg and1,100 Wh/l, respectively. However, the current Li-sulfur cells reportedby industry leaders in sulfur cathode technology have a maximum cellspecific energy of 250-350 Wh/kg (based on the total cell weight), whichis far below what is possible. In summary, despite its great potential,the practical realization of the Li—S battery has been hindered byseveral obstacles, such as dendrite-induced internal shorting, lowactive material utilization efficiency, high internal resistance,self-discharge, and rapid capacity fading on cycling. These technicalbarriers are due to the poor electrical conductivity of elementalsulfur, the high solubility of lithium polysulfides in organicelectrolyte (which migrate to the anode side, resulting in the formationof inactivated Li₂S in the anode), and Li dendrite formation andpenetration. The most serious problem remains to be the dendriteformation and penetration issues.

The traditional Na—S battery holds notable advantages, including highenergy density (theoretical value: 760 Wh/kg) and efficiency(approaching 100%), low material cost (rich abundances of Na and S innature), and long life. All these benefits make them promising forstationary storage applications, for example, utility-basedload-leveling and peak-shaving in smart grid, andemergency/uninterruptible power supply. However, this traditional Na—Smust operates at a temperature higher than 300° C. The ceramicelectrolyte is very brittle and, once a crack is initiated, the systemcan undergo a catastrophic failure, causing explosion. Thus, a Na—S cellthat operates at room temperature is highly desirable.

Sodium metal (Na) has similar chemical characteristics to Li and thesulfur cathode in room temperature sodium-sulfur cells (RT Na—Sbatteries) faces the same issues observed in Li—S batteries, such as:(i) low active material utilization rate, (ii) poor cycle life, and(iii) low Coulumbic efficiency. Again, these drawbacks arise mainly frominsulating nature of S, dissolution of polysulfide intermediates inliquid electrolytes (and related Shuttle effect), large volume changeduring charge/discharge, and dendrite penetration. Despite great effortsworldwide, dendrite formation and penetration remains the single mostcritical scientific and technological barrier against widespreadimplementation of all kinds of high energy density batteries having a Limetal anode.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials.Lithium metal would be an ideal anode material in a lithium-sulfursecondary battery if dendrite related issues could be addressed.

We have discovered a dendrite-resistant, nano graphene-enabled Li metalcell configuration [A. Zhamu, et al., “Reviving Rechargeable LithiumMetal Batteries: Enabling Next-Generation High-Energy or High-PowerCells,” Energy & Environment Science, 2012, 5, 5701-5707]. Each cellconsists of a graphene surface-supported Li metal anode and a cathodecontaining either graphene itself or a graphene-enhanced Li insertioncompound (e.g. vanadium oxide) as a cathode active material. Graphene isa single-atom thick layer of sp² carbon atoms arranged in ahoneycomb-like lattice. Graphene can be readily prepared from graphite,activated carbon, graphite fibers, carbon black, and meso-phase carbonbeads. Single-layer graphene and its slightly oxidized version (GO) canhave a specific surface area (SSA) as high as 2670 m²/g. It is this highsurface area that dramatically reduces the effective electrode currentdensity, which in turn significantly reduces the possibility of Lidendrite formation. More specifically, by implementing graphene sheetsto increase the anode surface areas, one can significantly reduce theanode current density, thereby dramatically prolonging the dendriteinitiation time and decreasing the growth rate of a dendrite, if everinitiated, possibly by a factor of up to 10¹⁰ and 10⁵, respectively.However, if a dendrite somehow is formed, this tree-like entity canquickly reach the separator and penetrate through it. It may be notedthat this nano-structured graphene layer is used to support a layer ofLi foil, which is disposed between the nano-structured graphene layerand the separator layer. There is nothing between the Li metal foillayer and the separator. This graphene nano-structure (behind the Limetal foil) is not capable of stopping or intercepting the dendrite onceformed.

Hence, an object of the present invention is to provide adendrite-stopping layer implemented between the Li or Na metal layer andthe porous separator layer in a rechargeable lithium metal battery orsodium metal battery that exhibits an exceptionally high specific energyor high energy density. One particular technical goal of the presentinvention is to provide a Li metal-sulfur or sodium-sulfur cell with acell specific energy greater than 500 Wh/Kg, preferably greater than 600Wh/Kg, and more preferably greater than 800 Wh/Kg (all based on thetotal cell weight). These must be accompanied by good resistance todendrite formation, and a long and stable cycle life. Thus, anotherobject of the present invention is to provide a simple, cost-effective,and easy-to-implement approach to preventing potential Li or Na metaldendrite-induced internal short circuit and thermal runaway problems inLi or Na metal-sulfur batteries.

SUMMARY OF THE INVENTION

The present invention provides dendrite penetration-resistant layer fora rechargeable alkali metal battery. This dendrite-stopping layercomprises an amorphous carbon or polymeric carbon matrix and particles(e.g. thin fibers or platelets) of an optional carbon or graphitereinforcement phase dispersed in the carbon matrix, which are chemicallybonded by a lithium- or or sodium-containing species to form an integrallayer that prevents dendrite penetration through this integral layer inthe intended alkali metal battery. The lithium- or sodium-containingspecies is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li,HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, LiSO_(y), Na₂CO₃, Na₂O,Na₂C₂O₄, NaOH, NaX, ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂, (CH₂OCO₂Na)₂,Na₂S, NaSO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=ahydrocarbon group, x=0-1, y=1-4; and wherein the lithium- orsodium-containing species is derived from an electrochemicaldecomposition reaction. The carbon matrix can be present alone withoutany reinforcement phase or filler (i.e. 100% carbon matrix). If areinforcement phase and/or a filler is present (dispersed in the carbonmatrix), then the amorphous carbon or polymeric carbon matrix and thereinforcement phase and/or the filler form a carbon matrix composite,wherein the matrix is preferably from 5% to 95% by volume (preferablyfrom 20% to 80% by volume and more preferably from 30% to 70% byvolume). The weight ratio of the carbon matrix to the lithium- orsodium-containing species can be varied from 1/100 to 100/1 (preferablyfrom 1/20 to 20/1 and more preferably from 1/10 to 10/1).

This dendrite penetration-resistant layer is disposed between an anodeactive material layer (e.g. a sheet of Li or Na foil) and a porousseparator layer wetted by a liquid or gel electrolyte (or a solidelectrolyte layer if the porous separator layer is not present).

The carbon or graphite reinforcement phase contains a material selectedfrom soft carbon particles, hard carbon particles, expanded graphiteflakes, carbon black particles, carbon nanotubes, carbon nano-fibers,carbon fibers, graphite fibers, polymer fibers, coke particles,meso-phase carbon particles, meso-porous carbon particles, electro-spunconductive nano fibers, carbon-coated metal nanowires, conductivepolymer-coated nanowires or nano-fibers, graphene sheets or platelets,or a combination thereof. This carbon or graphite reinforcement phase isused to strengthen the amorphous carbon or polymeric carbon matrix whichotherwise can be relatively weak. The amorphous carbon or polymericcarbon matrix is permeable to lithium ions or sodium ions. Thereinforcement phase is normally not permeable to lithium or sodium ionsunless this carbon or graphite material is intentionally made to containdefects (e.g. point defects, missing bonds, pores, etc.). Thus, a carbonmatrix containing a dispersed carbon/graphite reinforced phase makes anideal layer that, on one hand, stops dendrite penetration and, on theother hand, allows lithium and/or sodium ions to migrate through, whichis required for the battery to operate.

Such a rechargeable alkali metal battery (lithium metal battery orsodium metal battery) typically comprises: (A) an anode comprising analkali metal layer, an optional anode current collector layer, and thepresently invented dendrite penetration-resistant layer; (B) a cathodecomprising a cathode layer having a cathode active material forreversibly storing alkali metal ions and an optional cathode currentcollector layer to support the cathode active material; and (C) aseparator and electrolyte component in contact with the anode and thecathode; wherein the dendrite penetration-resistant layer is disposedbetween the alkali metal layer and the separator. Since any dendrite, ifpresent, would be stopped or intercepted by this dendritepenetration-resistant layer, the dendrite could not reach and penetratethe separator layer to cause internal shorting.

These lithium- or sodium-containing species are capable of bonding withthe carbon matrix and the carbon/graphite reinforcement phase to form astructurally sound layer that is sufficiently strong to intercept orstop dendrite penetration. Yet, such a layer is permeable to lithiumions or sodium ions. Preferably and typically, this layer iselectronically insulating, but ionically conducting (e.g. sodium ion- orlithium ion-conducting).

The carbon/graphite reinforcement phase may contain graphene sheets orplatelets including single-layer sheets or multi-layer platelets of agraphene material selected from pristine graphene, graphene oxide having2% to 46% by weight of oxygen, reduced graphene oxide having 0.01% to 2%by weight of oxygen, chemically functionalized graphene, nitrogen-dopedgraphene, boron-doped graphene, fluorinated graphene, or a combinationthereof and these graphene sheets or platelets are preferablyinterconnected (overlapped with one another). The graphene sheets orplatelets preferably have a thickness less than 10 nm. Preferably, thegraphene sheets or platelets contain single-layer or few-layer graphene,wherein few-layer is defined as 10 planes of hexagonal carbon atoms orless.

The amorphous carbon or polymeric carbon matrix is preferably obtainedby sputtering of carbon, chemical vapor deposition, chemical vaporinfiltration, or pyrolization of a polymer or pitch material.

In addition to or as an alternative to the carbon/graphite reinforcementphase, a filler (multiple particles) may be dispersed in the amorphouscarbon or polymeric carbon matrix. The filler may be selected from ametal oxide, metal carbide, metal nitride, metal boride, metaldichalcogenide, or a combination thereof. In an embodiment, the filleris selected from an oxide, dichalcogenide, trichalcogenide, sulfide,selenide, or telluride of niobium, zirconium, molybdenum, hafnium,tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese,iron, or nickel in a nanowire, nano-disc, nano-ribbon, or nano plateletform.

In a preferred embodiment, the filler is selected from nano discs, nanoplatelets, or nano sheets of an inorganic material selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, cobalt, manganese, iron, nickel, or a transition metal; (d)boron nitride, or (e) a combination thereof; wherein the discs,platelets, or sheets have a thickness less than 100 nm. These 2D nanomaterials are found to be very effective in stopping dendritepenetration; however, they are normally not very permeable to lithiumions or sodium ions. Hence, they must be dispersed in a carbon matrixthat is permeable to lithium or sodium ions.

There is no restriction on the thickness of the dendrite-interceptinglayer, but for practical purposes, the dendrite penetration-resistantlayer preferably has a thickness from 10 nm to 20 μm, more preferablyfrom 100 nm to 10 μm, and most preferably from 100 nm to 5 μm. In onepreferred embodiment, the dendrite penetration-resistant layer is alithium ion conductor or sodium ion conductor having an ion conductivityno less than 10⁻⁴ S/cm, more preferably no less than 10⁻³ S/cm.

The present invention also provides a process for producing such adendrite penetration-resistant layer. The process comprises: (a)preparing a working electrode containing a structure (layer) of anamorphous carbon or polymeric carbon matrix and an optional carbon orgraphite reinforcement phase dispersed in the carbon matrix; (b)preparing a counter electrode containing lithium or sodium metal oralloy; (c) bringing the working electrode and the counter electrode incontact with an electrolyte containing a solvent and a lithium salt orsodium salt dissolved in this solvent; and (d) applying a current orvoltage to the working electrode and the counter electrode to induce anelectrochemical oxidative decomposition and/or a reductive decompositionof the electrolyte for forming the lithium- or sodium-containing speciesthat are chemically bonded to the amorphous carbon or polymeric carbonmatrix and/or the optional carbon or graphite reinforcement phase toproduce the dendrite penetration-resistant layer.

The lithium salt or sodium salt in this electrochemical decompositionreactor is selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), sodium borofluoride(NaBF₄), sodium hexafluoroarsenide, sodium trifluoro-metasulfonate(NaCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), or a combination thereof. It may benoted that these alkali metal salts can also be used in the electrolytethat is part of the intended alkali metal secondary battery.

The solvent in this electrochemical reactor may be selected from1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), ahydrofluoroether, an ionic liquid solvent, or a combination thereof. Itmay be further noted that these solvents can also be used in theelectrolyte that is part of the intended alkali metal secondary battery.

The electrochemical decomposition treatment may be carried out in aroll-to-roll manner. In an embodiment, the continuous-length layer ofcarbon nanotube/graphene-reinforced carbon composite structure (e.g.CNT/graphene paper infiltrated with CVD carbon) may be unwound from afeeder roller, and moved to enter an electrochemical treatment zone(equivalent to an electrochemical decomposition reactor) containing anelectrolyte therein. Immersed in this electrolyte is a lithium or sodiumelectrode and the graphene paper is also electrically wired as theworking electrode. The carbon-infiltrated CNT/graphene paper is moved ata controlled speed to give enough time for electrochemical decompositionof the electrolyte to occur. The paper is impregnated with and/or bondedby the decomposition products and the product is then wound up on atake-up roller. This roll-to-roll or reel-to-reel process can be easilyscaled up and automated for mass production of the presently inventeddendrite penetration resistant layer products.

In other words, in an embodiment, the process is a roll-to-roll processthat includes preparing the working electrode in a roll form supportedby a roller, and the step of bringing the working electrode and thecounter electrode in contact with the electrolyte contains unwinding theworking electrode from the roller, and feeding the working electrodeinto the electrolyte.

In an alternative embodiment, a sheet of carbon matrix composite papermay be unwound from a feeder roller, deposited with some lithium orsodium metal (e.g. using physical vapor deposition or sputtering) whilethe paper is in a dry state. The Li- or Na-deposited carbon matrixcomposite paper is then moved to enter an electrochemical treatment zonecontaining an electrolyte therein. As soon as the Li-carbon compositelayer or Na-carbon composite layer enters the electrolyte, essentiallyshort-circuiting occurs between the carbon composite and Li (or Na). Inother words, the carbon composite “electrode” is essentially placed inan electrochemical potential that is 0 V with respect to Li⁺/Li orNa⁺/Na, subjecting the electrolyte to a reductive decomposition andenabling decomposition products to react with the carbon matrixcomposite in situ. Optionally a lithium or sodium electrode isimplemented and immersed in this electrolyte and the carbon compositepaper is electrically wired as the working electrode. Such anarrangement aids in continuing electrochemical decomposition ofelectrolytes and formation of the bonding Li- or Na-containing species.The carbon matrix composite is impregnated with and/or bonded by thedecomposition products, which product is then wound up on a take-uproller.

Thus, an alternative process for producing the dendritepenetration-resistant layer comprises (a) preparing a working electrodecontaining a layer of the carbon matrix or carbon matrix composite; (b)preparing a counter electrode containing lithium or sodium metal oralloy; and (c) bringing the working electrode and the counter electrodein physical contact with each other and in contact with an electrolytecontaining a solvent and a lithium salt or sodium salt dissolved in thesolvent; wherein the working electrode and the counter electrode arebrought to be at the same electrochemical potential level, inducing achemical reaction between the lithium/sodium metal or alloy and thecarbon matrix or carbon matrix composite, and inducing electrochemicaldecomposition of the electrolyte for forming the lithium- orsodium-containing species that are chemically bonded to the carbonmatrix and/or the reinforcement phase to produce the dendritepenetration-resistant layer either outside of or inside an intendedrechargeable alkali metal battery. In an embodiment, this process isconducted in a roll-to-roll manner outside of the intended rechargeablealkali metal battery. Alternatively, this process is conducted insidethe intended rechargeable alkali metal battery; the battery itself isregarded as an electrochemical decomposition reactor.

In an alternative embodiment, a process for producing a dendritepenetration-resistant layer is herein provided. This process comprises:(a) preparing an alkali metal battery cell comprising an anode alkalimetal layer, a layer of carbon matrix or carbon matrix composite, aporous separator layer, and a cathode layer, wherein the layer of carbonmatrix or carbon matrix composite is laminated between the alkali metallayer and the porous separator layer and the porous separator layer isdisposed between the layer of carbon matrix or carbon matrix compositeand the cathode layer; and (b) subjecting the battery cell to avoltage/current treatment that induces electrochemical reductive and/oroxidative decomposition to form the lithium- and/or sodium-containingspecies that are chemically bonded to the carbon matrix or carbon matrixcomposite to form the dendrite penetration-resistant layer inside thisbattery cell. In an embodiment, the step (a) of preparing an alkalimetal battery cell comprises dispensing and depositing carbon matrix orcarbon matrix composite onto the alkali metal layer to form a layer upto a thickness from 2 nm to 20 μm. This layer of carbon matrix or carbonmatrix composite is ultimately covered by or laminated with the porousseparator layer.

The cathode active material in this rechargeable alkali metal batterymay be selected from sulfur, selenium, tellurium, lithium sulfide,lithium selenide, lithium telluride, sodium sulfide, sodium selenide,sodium telluride, a chemically treated carbon or graphite materialhaving an expanded inter-graphene spacing d₀₀₂ of at least 0.4 nm, or anoxide, dichalcogenide, trichalcogenide, sulfide, selenide, or tellurideof a transition metal, such as niobium, zirconium, molybdenum, hafnium,tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese,iron, nickel, or a combination thereof.

In an embodiment, the cathode layer contains an air cathode and thebattery is a lithium-air battery or sodium-air battery. In anotherembodiment, the cathode active material is selected from sulfur orlithium polysulfide and the battery is a lithium-sulfur or sodium-sulfurbattery.

The electrolyte in the intended alkali metal secondary battery may beselected from polymer electrolyte, polymer gel electrolyte, compositeelectrolyte, ionic liquid electrolyte, aqueous electrolyte, non-aqueousliquid electrolyte, soft matter phase electrolyte, solid-stateelectrolyte, or a combination thereof.

The alkali metal layer in the anode may contain an anode active materialselected from lithium metal, sodium metal, a lithium metal alloy, sodiummetal alloy, a lithium intercalation compound, a sodium intercalationcompound, a lithiated compound, a sodiated compound, or a combinationthereof. The Li or Na content in this alkali metal layer preferably isat least 70% by weight, more preferably >80%, and most preferably >90%.

The advantages and features of the present invention will become moretransparent with the description of the following best mode practice andillustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a rechargeable lithium or sodium battery featuring adendrite-stopping layer implemented between a lithium or sodium metallayer and a porous separator layer.

FIG. 2 Schematic of the most commonly used procedures for producingexfoliated graphite worms and graphene sheets;

FIG. 3 Another schematic drawing to illustrate the process for producingexfoliated graphite, expanded graphite flakes, and graphene sheets.

FIG. 4 SEM images of exfoliated graphite worms imaged at a lowmagnification;

FIG. 5 TEM image of single-layer graphene sheets partially stackedtogether.

FIG. 6 An energy diagram to illustrate electrochemical potential orenergetic conditions under which electrolyte in an electrochemicalreactor undergoes oxidative or reductive degradation at theelectrode-electrolyte boundary.

FIG. 7 The specific discharge capacities of two Li—S cells, onecontaining the presently invented dendrite-intercepting layer and theother not containing such a layer, are plotted as a function of thenumber of charge/discharge cycles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For illustration purpose, the following discussion of preferredembodiments is primarily based on Li—S cells (as an example), but thesame or similar principles and procedures are applicable to all otherrechargeable lithium metal batteries (using lithium metal or metal alloyas the anode active material) and all rechargeable sodium metalbatteries (using sodium metal or metal alloy as the anode activematerial). The cathode active materials can be, for instance, atransition metal oxide (e.g. V₂O₅) or sulfide (e.g. MoS₂), sulfur orpolysulfide (e.g. lithium polysulfide or sodium polysulfide), or justoutside air (for a lithium-air or sodium-air battery).

The present invention provides a dendrite penetration resistant layer tobe implemented between an anode active material layer (e.g. a Li foil orNa foil) and a porous separator impregnated with liquid or gelelectrolyte (or a solid electrolyte), as illustrated in FIG. 1. In apreferred embodiment, such a dendrite-intercepting or dendrite-stoppinglayer is made from an integral layer of carbon matrix or carbon matrixcomposite containing a reinforcement phase (e.g. a carbon or graphitefiber) and/or a filler (e.g. inorganic 2D nano materials in the form ofa nano disc, nano platelet, nano sheet, nano belt, or nano ribbon).There are typically gaps or voids between nanosheets/platelets/discs/ribbons. These gaps or voids are impregnated by acarbon matrix material and chemically active lithium- and/orsodium-containing species that chemically bond to the edges and/or facesof these sheets/platelets/discs/ribbons, essentially sealing off most orall of these gaps. Due to the high strength of the integral layer, adendrite, if existing and growing, cannot penetrate through thisintegral layer to reach the separator layer. However, these nanosheets/platelets/discs/ribbons (if containing a good amount of pointdefects) and the lithium- or sodium-containing species can be made to bepermeable to lithium ions or sodium ions. The amorphous carbon andpolymeric carbon matrix itself is highly permeable to lithium and sodiumions.

These lithium- or sodium-containing bonding species can be simply theproducts or by-products of chemical reactions between the electrolyte(Li or Na salt and solvent) and the carbon matrix and/or thecarbon/graphite-based reinforcement phase that are induced by externallyapplied current/voltage in an electrochemical reactor. This will bediscussed in more detail later.

In a preferred embodiment, the lithium- or sodium-containing species maybe selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi,(ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), Na₂CO₃, Na₂O, Na₂C₂O₄,NaOH, NaX, ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂, (CH₂OCO₂Na)₂, Na₂S,Na_(x)SO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=ahydrocarbon group (e.g. R=CH—, CH₂—, CH₃CH₂—, etc.), x=0-1, y=1-4. Thesespecies are surprisingly capable of bonding a wide variety ofcarbon/graphite reinforcement materials and/or carbon matrix together toform a structurally sound layer that is sufficiently strong to interceptor stop dendrite penetration. Carbon matrix composite can contain agraphite/carbon reinforcement material selected from multiplesheets/platelets of a graphene material, multiple flakes of exfoliatedgraphite, carbon nano-fibers, carbon nanotubes, carbon fibers, graphitefibers, carbon black or acetylene black particles, needle coke, softcarbon particles, hard carbon particles, artificial graphite particles.These particle or fibers preferably have a diameter or thickness lessthan 10 μm, preferably less than 1 μm, further preferably less than 200nm, and most preferably less than 100 nm. Such a carbon matrix compositelayer is also permeable to lithium ions or sodium ions. Preferably, thislayer is electronically insulating, but ionically conducting. Typically,not just one, but at least two types of lithium- or sodium-containingspecies in the above list are present in the dendritepenetration-resistant layer.

A. Production of Thin Films of Carbon Matrix and Carbon MatrixComposites

The carbon matrix (without a graphite/carbon reinforcement phase orfiller) can be produced by several processes. For instance, thin filmsof amorphous carbon can be deposited on a solid substrate surface usingchemical vapor deposition of hydrocarbon gas introduced into a chamberat a temperature of 400-1,200° C. under a hydrogen or noble gasatmosphere. Alternatively, amorphous carbon can be produced bysputtering of carbon atoms or clusters of C atoms onto a solid substratesurface from a carbon target in a vacuum chamber. The resultingamorphous carbon films can then be peeled off from the substrate toobtain free-standing films.

Carbon films may also be produced by pyrolyzation of polymer films(including thermoplastic films, thermoset films, coal tar pitch films,petroleum pitch films, etc., free-standing or coated on a solidsurface), typically at an initial oxidation temperature of 250-350° C.(e.g. for polyacrylonitrile, PAN), followed by a carbonization treatmentat 500-1,500° C. For other polymer films, heat treatments can godirectly into the range of 500-1,500° C. without a pre-oxidation (e.g.phenolic resin). These films are herein referred to as polymeric carbonor carbonized resin films. There is no restriction on the kind ofpolymer or pitch material that can be pyrolyzed to produce the neededcarbon matrix; but, preferably, the resin or pitch has a carbon yield ofat least 20% (more preferably at least 30% and most preferably from 40%to approximately 75%).

Thin films of a polymer matrix composite (e.g. a mixture of phenolicresin+CNTs and/or graphene sheets) can be prepared in a free-standingform or coated on a solid substrate. This can be made by a solventmixing or melt mixing procedure that is well-known in the art. Thisresin matrix composite is then subjected to the heat treatments asdescribed above (e.g. at a temperature in the range of 500-1,500° C.) toobtain carbon matrix composites.

Alternatively, one can prepare a sheet of porous non-woven, mat, paper,foam, or membrane of a carbon/graphite reinforcement material (e.g.graphene sheets, expanded graphite flakes, CNTs, carbon nano-fibers,etc.) by using any known process. This porous structure is theninfiltrated with carbon using chemical vapor deposition (CVD),sputtering, or chemical vapor infiltration (CVI) to obtain a carbonmatrix composite. Further alternatively, this porous structure can beimpregnated with a resin or pitch material and the resulting compositebe pyrolyzed to obtain a carbon matrix composite.

As a graphite/carbon reinforcement material, graphene sheets orplatelets can be selected from single-layer sheets or multi-layerplatelets of a graphene material selected from pristine graphene,graphene oxide having 2% to 46% by weight of oxygen, reduced grapheneoxide having 0.01% to 2% by weight of oxygen, chemically functionalizedgraphene, nitrogen-doped graphene, boron-doped graphene, fluorinatedgraphene, or a combination thereof and these graphene sheets orplatelets are preferably interconnected (overlapped with one another).The graphene sheets or platelets preferably have a thickness less than10 nm, more preferably less than 2 nm. Preferably, the graphene sheetsor platelets contain single-layer or few-layer graphene, whereinfew-layer is defined as 10 planes of hexagonal carbon atoms or less.Preferably, the graphene planes have a controlled amount of pointdefects (e.g. missing C atoms, incomplete carbon hexagon structures,etc.), which are fast paths for migration of lithium or sodium ions.These point defects are typically residues of what used to be chemicalfunctional groups (e.g. —C══O, —OH, —COOH, —NH₂, —O—, —F, —Cl, —Br, —I,etc.) originally attached to graphene planes.

The graphene sheets or platelets or exfoliated graphite flakespreferably have a length or width smaller than 1 μm, preferably smallerthan 0.5 μm, more preferably smaller than 200 nm, and most preferablysmaller than 100 nm. These desired dimensions are measured before thesesheets/platelets/flakes are bonded by the lithium- or sodium-containingspecies. We have unexpectedly discovered that smaller graphene sheetsnormally lead to higher ion conductivity values, beneficial to ratecapabilities of the battery.

As a graphite/carbon reinforcement material, CNTs can be single-walledor multi-walled. Both CNTs and CNFs (carbon nano-fibers), as well asother hard carbon, soft carbon, needle coke, and carbon black particles,can be chemically etched to produce defects that allow for easierpermeation of sodium or lithium ions. Further, these carbon/graphitematerial can be chemically functionalized to attach desired chemicalfunctional groups (e.g. —C══O, —OH, —COOH, —NH₂, —O—, —F, —Cl, —Br, —I,etc.) to the ends/surfaces of these nanotubes or nano-fibers.

The carbon matrix films or carbon matrix composite films can be solid orporous. The pores eventually will be substantially filled with lithium-or sodium-containing species to form a layer of structural integrity.These lithium- or sodium-containing species are surprisingly capable ofchemically bonding to the carbon matrix or the carbon/graphitereinforcement particles/fibers/nanotubes, forming a layer of lithium orsodium ion-conducting structure that is strong and tough.

In addition to or as an alternative to the carbon/graphite reinforcementphase, a filler in the form of multiple particles may be dispersed inthe amorphous carbon or polymeric carbon matrix. The filler may beselected from a metal oxide, metal carbide, metal nitride, metal boride,metal dichalcogenide, or a combination thereof. In an embodiment, thefiller is selected from an oxide, dichalcogenide, trichalcogenide,sulfide, selenide, or telluride of niobium, zirconium, molybdenum,hafnium, tantalum, tungsten, titanium, vanadium, chromium, cobalt,manganese, iron, or nickel in a nanowire, nano-disc, nano-ribbon, ornano platelet form.

Preferably, the filler is selected from nano discs, nano platelets, ornano sheets of an inorganic material selected from: (a) bismuth selenideor bismuth telluride, (b) transition metal dichalcogenide ortrichalcogenide, (c) sulfide, selenide, or telluride of niobium,zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt,manganese, iron, nickel, or a transition metal; (d) boron nitride, or(e) a combination thereof; wherein the discs, platelets, or sheets havea thickness less than 100 nm. These 2D nano materials are found to bevery effective in helping to stop dendrite penetration; however, theyare normally not very permeable to lithium ions or sodium ions. Hence,they must be dispersed in a carbon matrix that is permeable to lithiumor sodium ions.

When a non-carbon/non-graphite filler exists in the carbon matrix, thefiller amount is preferably <50% by volume, more preferably <30% byvolume, and most preferably <20% by volume. When the filler and/or thecarbon/graphite reinforcement phase is present in the carbon matrix, thematrix is preferably from 5% to 95% by volume (preferably from 20% to80% by volume and more preferably from 30% to 70% by volume). The weightratio of the carbon matrix to the lithium- or sodium-containing speciescan be varied from 1/100 to 100/1

B. Methods or Processes for Producing Lithium- or Sodium-ContainingSpecies

The preparation of dendrite-stopping layers may be conducted in anelectrochemical reactor, which is an apparatus very similar to anelectrode plating system. In this reactor, an amorphous carbon orpolymeric carbon matrix (with or without a carbon/graphite reinforcementmaterial), in the form of a mat, paper, film, etc., is used as a workingelectrode and lithium sheet (or sodium sheet) as a counter electrode.Contained in the reactor is an electrolyte composed of a lithium orsodium salt dissolved in a solvent (e.g. 1M LiPF₆ dissolved in a mixtureof ethylene carbonate (EC) and dimethyl carbonate (DMC) at a 1:1 ratioby volume). A current is then imposed between these two electrodes(lithium or sodium sheet electrode and the carbon working electrode).The carbon matrix and the carbon/graphite reinforcement material in theworking electrode are galvanostatically discharged (e.g. Li ions beingsent to and captured by these carbon matrix and/or carbon/graphitereinforcement materials) and charged (Li ions released by thesecarbon/graphite materials) in the voltage range from 0.01V to 4.9V atthe current densities of 100-1000 mA/g following a voltage-currentprogram similar to what would be used in a lithium-ion battery. However,the system is intentionally subjected to conditions conducive tooxidative degradation of electrolyte (e.g. close to 0.01-1.0 V vs.Li/Li⁺) or reductive degradation of electrolyte (4.1-4.9 V vs. Li/Li⁺)for a sufficient length of time. The degradation products react with Li⁺ions, Li salt, functional groups (if any) or carbon atoms on/in thecarbon matrix or carbon/graphite reinforcement to form thelithium-containing species that also chemically bond to the carbonmatrix or composite. Sodium-containing bonding species can be formed ina similar manner by using a sodium sheet as the counter-electrode and asodium salt, alone or in combination with a lithium salt, is dissolvedin the solvent to make a liquid electrolyte. Other electrochemicaltreatment conditions follow the same basic principles.

The chemical compositions of the lithium-containing species are governedby the voltage range, the number of cycles (from 0.01 V to 4.9 V, andback), solvent type, lithium salt type, chemical composition ofcarbon/graphite phase (e.g. % of O, H, and N attached to CNTs, CNFs,exfoliated graphite flakes, graphene sheets, etc.), and electrolyteadditives (e.g. LiNO₃, if available). The morphology, structure andcomposition of carbon/graphite reinforcement phase, the amorphous carbonmatrix, the lithium-containing species that are bonded to the carbonmatrix and/or reinforcement phases can be characterized by scanningelectron microscope (SEM), transmission electron microscope (TEM), Ramanspectrum, X-ray diffraction (XRD), Fourier Transform InfraredSpectroscopy (FTIR), elemental analysis, and X-ray photoelectronspectroscopy (XPS).

The decomposition of non-aqueous electrolyte leads to the formation oflithium or sodium chemical compounds that bond to surface/ends of CNTs,graphene surfaces and edges, functional groups of chemically treatedcarbon black particles, etc. The reasons why the non-aqueous electrolyteis decomposed during discharge-charge cycling in an electrochemicalreactor may be explained as follows. As illustrated in FIG. 6, in anelectrochemical reactor system where there are a cathode and an anode incontact with an electrolyte, the thermodynamic stability of theelectrolyte is dictated by the relative electron energies of the twoelectrodes relative to the energy level of the non-aqueous electrolyte.The anode is potentially a reductant, and the cathode an oxidant. Thetwo electrodes are typically electronic conductors and, in this diagram,their electrochemical potentials are designated as μ_(A) and μ_(C) (orFermi energies ε_(F)), respectively. The energy separation, E_(g),between the lowest unoccupied molecular orbital (LUMO) and the highestoccupied molecular orbital (HOMO) of the electrolyte is the stableelectrochemical window of the electrolyte. In other words, in order forthe electrolyte to remain thermodynamically stable (i.e. not todecompose), the electrochemical potential of the anode (μ_(A)) must bemaintained below the LOMO and μ_(C) of the cathode must be above theHOMO.

From the schematic diagram of FIG. 6, we can see that an anode withμ_(A) above the LUMO and a cathode with μ_(C) below the HOMO will reduceand oxidize the electrolyte, respectively, unless a passivating film isformed that creates a barrier to electron transfer between the anode andelectrolyte or between the cathode and the electrolyte. In the presentlyinvented method, an external current/voltage is intentionally appliedover the anode and the cathode to bias their respective electrochemicalpotential levels so that the electrolyte can go outside of the stableelectrochemical potential window, undergoing oxidative and/or reductivedegradation. The degradation products are reactive species that reactamong themselves and with the functional groups or active atoms ofcarbon matrix and/or carbon/graphite reinforcement phase, forming a massof lithium- or sodium-containing species that bond the carbon matrix andthe reinforcement phase materials together.

For the list of lithium/sodium salts and solvents investigated, theelectrolytes have an oxidation potential (HOMO) at about 4.7 V and areduction potential (LUMO) near 1.0 V. (All voltages in thisspecification are with respect to Li⁺/Li or Na⁺/Na). We have observedthat the chemical interaction of Li⁺ or Na⁺ ions with graphene planes oredges occur at about 0.01-0.8 V, so electrolytes are prone to reductivedegradation in the voltage range of 0.01-0.8 V. By imposing a voltageclose to 4.7 volts, the electrolytes are also subject to oxidativedegradation. The degradation products spontaneously react with chemicalspecies associated with the carbon matrix and/or reinforcement materials(e.g. graphene planes or edges), forming a material phase that bondstogether with carbon matrix and/or reinforcement materials during thecharge-discharge cycling (electrolyte reduction-oxidation cycling). Ingeneral, these lithium- or sodium-containing species are notelectrically conducting and, hence, these reactions can self-terminateto form essentially a passivating phase.

The electrolytes that can be used in this electrochemical decompositionreactor may be selected from any lithium or sodium metal salt that isdissolvable in a solvent to produce an electrolyte. Preferably, themetal salt is selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), sodium perchlorate(NaClO₄), sodium hexafluorophosphate (NaPF₆), sodium borofluoride(NaBF₄), sodium trifluoro-metasulfonate (NaCF₃SO₃), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide(NaTFSI), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂), or acombination thereof. It may be noted that these metal salts are alsocommonly used in the electrolytes of rechargeable lithium or sodiumbatteries.

The electrolytes used in this electrochemical reactor may contain asolvent selected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofluoroether, a room temperatureionic liquid solvent, or a combination thereof. These solvents are alsocommonly used in the electrolytes of rechargeable lithium or sodiumbatteries.

C. Production of Various Carbon/Graphite Reinforcement Materials

Carbon/graphite-based reinforcement materials that can be used tostrengthen the carbon matrix in the dendrite-stopping layer includecarbon nanotubes (CNTs), carbon nano-fibers (CNFs), graphenesheets/platelets, expanded graphite flakes, fine particles of carbonblack (CB) or acetylene black (AB), needle coke, etc. These speciesshould preferably have a diameter or thickness less than 1 μm,preferably less than 500 nm, more preferably less than 200 nm, and mostpreferably less than 100 nm. Traditional carbon fibers or graphitefibers, having a diameter of typically 6-12 μm, are not preferredchoices if one desires to make a dendrite-stopping layer thinner than 10μm for the purpose of reducing battery volume and weight.

Most of these materials are commercially available. Some of thepreferred reinforcement materials and their treatments are hereindescribed. In a preferred embodiment, the graphene sheets in adendrite-intercepting layer is selected from pristine graphene, grapheneoxide, reduced graphene oxide, graphene fluoride, graphene chloride,graphene bromide, graphene iodide, hydrogenated graphene, nitrogenatedgraphene, chemically functionalized graphene, or a combination thereof.Alternatively, the backbone of a dendrite-intercepting layer may beselected from flakes of an exfoliated graphite material. The startinggraphitic material for producing any one of the above graphene orexfoliated graphite materials may be selected from natural graphite,artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbonmicro-bead, soft carbon, hard carbon, coke, carbon fiber, carbonnano-fiber, carbon nano-tube, or a combination thereof.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of hexagonal carbon atoms,which are single-atom thick, provided the inter-planar van der Waalsforces can be overcome. An isolated, individual graphene plane of carbonatoms is commonly referred to as single-layer graphene. A stack ofmultiple graphene planes bonded through van der Waals forces in thethickness direction with an inter-graphene plane spacing ofapproximately 0.3354 nm is commonly referred to as a multi-layergraphene. A multi-layer graphene platelet has up to 300 layers ofgraphene planes (<100 nm in thickness), but more typically up to 30graphene planes (<10 nm in thickness), even more typically up to 20graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs).Graphene sheets/platelets (collectively, NGPs) are a new class of carbonnano material (a 2-D nano carbon) that is distinct from the 0-Dfullerene, the 1-D CNT or CNF, and the 3-D graphite. For the purpose ofdefining the claims and as is commonly understood in the art, a graphenematerial (isolated graphene sheets) is not (and does not include) acarbon nanotube (CNT) or a carbon nano-fiber (CNF).

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004) (US Patent PublicationNo. 2005/0271574A1 now abandoned); and (3) B. Z. Jang, A. Zhamu, and J.Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,”U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006) (US PatentPublication No. 2008/0048152A1 now abandoned).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 2 and FIG. 3 (schematic drawings). The presenceof chemical species or functional groups in the interstitial spacesbetween graphene planes in a GIC or GO serves to increase theinter-graphene spacing (d₀₀₂, as determined by X-ray diffraction),thereby significantly reducing the van der Waals forces that otherwisehold graphene planes together along the c-axis direction. The GIC or GOis most often produced by immersing natural graphite powder (100 in FIG.3) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), andanother oxidizing agent (e.g. potassium permanganate or sodiumperchlorate). The resulting GIC (102) is actually some type of graphiteoxide (GO) particles if an oxidizing agent is present during theintercalation procedure. This GIC or GO is then repeatedly washed andrinsed in water to remove excess acids, resulting in a graphite oxidesuspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. In order toproduce graphene materials, one can follow one of the two processingroutes after this rinsing step, briefly described below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. application Ser. No. 10/858,814 (Jun. 3,2004) (now abandoned). Single-layer graphene can be as thin as 0.34 nm,while multi-layer graphene can have a thickness up to 100 nm, but moretypically less than 10 nm (commonly referred to as few-layer graphene).Multiple graphene sheets or platelets may be made into a sheet of NGPpaper using a paper-making process. This sheet of NGP paper is anexample of the porous graphene structure layer utilized in the presentlyinvented process.

Route 2 entails ultrasonicating the graphite oxide suspension (e.g.graphite oxide particles dispersed in water) for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation has been increased from 0.3354 nm in natural graphiteto 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form fully separated, isolated, or discrete graphene oxide(GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% by weightof oxygen.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Pristine graphene, in smaller discrete graphene sheets (typically 0.3 μmto 10 μm), may be produced by direct ultrasonication (also known asliquid phase exfoliation or production) or supercritical fluidexfoliation of graphite particles. These processes are well-known in theart.

The graphene oxide (GO) may be obtained by immersing powders orfilaments of a starting graphitic material (e.g. natural graphitepowder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid,nitric acid, and potassium permanganate) in a reaction vessel at adesired temperature for a period of time (typically from 0.5 to 96hours, depending upon the nature of the starting material and the typeof oxidizing agent used). As previously described above, the resultinggraphite oxide particles may then be subjected to thermal exfoliation orultrasonic wave-induced exfoliation to produce isolated GO sheets. TheseGO sheets can then be converted into various graphene materials bysubstituting —OH groups with other chemical groups (e.g. —Br, NH₂,etc.).

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished.

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF),carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F), onlyhalf of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers of graphene plane, it is necessary to overcome theattractive forces between adjacent layers and to further stabilize thelayers. This may be achieved by either covalent modification of thegraphene surface by functional groups or by non-covalent modificationusing specific solvents, surfactants, polymers, or donor-acceptoraromatic molecules. The process of liquid phase exfoliation includesultra-sonic treatment of a graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

It may be noted that graphene oxide, graphene fluoride, graphenebromide, nitrogenated graphene, doped graphene (e.g. B-doped graphene),etc. contain many functional groups that can become defects on grapheneplanes when these groups are removed. These functional groups, whenremoved, result in the formation of point defects, which could promotemigration of lithium or sodium ions.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 3, a graphite particle (e.g.100) is typically composed of multiple graphite crystallites or grains.A graphite crystallite is made up of layer planes of hexagonal networksof carbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection). A highly ordered graphite particle can consist ofcrystallites of a considerable size, having a length of L_(a) along thecrystallographic a-axis direction, a width of L_(b) along thecrystallographic b-axis direction, and a thickness L_(c) along thecrystallographic c-axis direction.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 3) are intercalated in an acid solution to produce graphiteintercalation compounds (GICs, 102). The GICs are washed, dried, andthen exfoliated by exposure to a high temperature for a short period oftime. This causes the flakes to expand or exfoliate in the c-axisdirection of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as graphite worms 104. These wormsof graphite flakes which have been greatly expanded can be formedwithout the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications. Examples ofexfoliated graphite worms (or, simply, graphite worms) are presented inFIG. 4.

Acids, such as sulfuric acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planesto obtain GICs. Many other types of intercalating agents, such as alkalimetals (Li, K, Na, Cs, and their alloys or eutectics), can be used tointercalate graphite to stage 1, stage 2, stage 3, etc. Stage n impliesone intercalant layer for every n graphene planes. For instance, astage-1 potassium-intercalated GIC means there is one layer of K forevery graphene plane; or, one can find one layer of K atoms insertedbetween two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence,where G is a graphene plane and K is a potassium atom plane. A stage-2GIC will have a sequence of GG/K/GG/K/GG/K/GG and a stage-3 GIC willhave a sequence of GGG/K/GGG/K/GGG . . . , etc. These GICs can then bebrought in contact with water or water-alcohol mixture to produceexfoliated graphite and/or separated/isolated graphene sheets.

D. Production of Integral Layer of Porous Carbon/Graphite ReinforcementStructure

Several techniques can be employed to fabricate a conductive layer ofporous carbon/graphite reinforcement structure (a web, mat, paper,non-woven, foam, or porous film, etc.), which is a monolithic bodyhaving desired interconnected pores that are accessible to carbon matrixto be infiltrated later. Some residual pores after carbon matrixinfiltration can remain accessible to the liquid electrolyte in anelectrochemical decomposition reactor. These electrolytes are to beintentionally decomposed under the oxidation-reduction cyclingconditions discussed earlier in Section B to form the desired lithium-or sodium-containing species that are chemically bonded to the carbonmatrix and/or the carbon/graphite reinforcement materials (e.g. CNTs,graphene sheets, etc.).

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is heavily re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (106 in FIG. 3), which aretypically 100-500 μm thick. Even though the flexible graphite foil isporous, most of these pores are not accessible to carbon matrix through,for instance, chemical vapor infiltration (CVI) or resin infiltrationand carbonization of resin. For the preparation of a desired layer ofporous graphene structure (optionally along with other carbon/graphitematerials), the compressive stress and/or the gap between rollers can bereadily adjusted to obtain a desired layer of porous graphene structurethat has pores accessible to liquid electrolyte.

Exfoliated graphite worms may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets (NGPs) with all the graphene platelets thinner than100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 3). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms. Amass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide) may be madeinto a graphene film/paper (114 in FIG. 3) using a film- or paper-makingprocess.

Alternatively, with a low-intensity shearing, graphite worms tend to beseparated into the so-called expanded graphite flakes (108 in FIG. 3)having a thickness >100 nm. These flakes can be formed into exfoliatedgraphite paper or mat 106 using a paper- or mat-making process, with orwithout a resin binder. In one preferred embodiment of the presentinvention, the porous web can be made by using a slurry molding or aflake/binder spraying technique. These methods can be carried out in thefollowing ways:

As a wet process, aqueous slurry is prepared which comprises a mixtureof graphene sheets or expanded graphite flakes and, optionally, about0.1 wt. % to about 10 wt. % resin powder binder (e.g., phenolic resin),which will be carbonized later. The slurry is then directed to impingeupon a sieve or screen, allowing water to permeate through, leavingbehind sheets/flakes and the binder. As a dry process, the directedsheet/flake spray-up process utilizes an air-assisted flake/binderspraying gun, which conveys flakes/sheets and an optional binder to amolding tool (e.g., a perforated metal screen shaped identical orsimilar to the part to be molded). Air goes through perforations, butthe solid components stay on the molding tool surface. The porouscarbon/graphite reinforcement structure can be infiltrated directly withCVI carbon or indirectly with a resin or pitch material, followed bycarbonization of the pitch or resin.

Each of these routes can be implemented as a continuous process in aroll-to-roll manner. For instance, the process begins with pulling asubstrate (porous sheet) from a roller. The moving substrate receives astream of slurry (as described in the above-described slurry moldingroute) from above the substrate. Water sieves through the poroussubstrate with all other ingredients (a mixture of graphene sheets,graphite flakes, and/or CNFs, etc. an optional filler, and an optionalbinder) remaining on the surface of the substrate being moved forward togo through a compaction stage by a pair of compaction rollers. Heat maybe supplied to the mixture before, during, and after compaction to helpcure the thermoset binder for retaining the shape of the resulting webor mat. The web or mat, with all ingredients held in place by thethermoset binder, may be stored first (e.g., wrapped around a roller).Similar procedures may be followed for the case where the mixture isdelivered to the surface of a moving substrate by compressed air, likein a directed fiber/binder spraying process. Air will permeate throughthe porous substrate with other solid ingredients trapped on the surfaceof the substrate, which are conveyed forward. The subsequent operationsare similar than those involved in the slurry molding route. The resinbinder, if present, is preferably burned off from the graphene sheetframework before the porous graphene structure is implemented in anelectrochemical decomposition reactor.

Alternatively, rolls of porous carbon/graphite paper/mat may be readilyproduced in a cost-effective manner using other well-known paper-making,foam-making, or mat-making techniques, etc.

The porous structure (e.g. paper, mat, foam, etc.) is then subjected tocarbon infiltration via direct CVI or liquid infiltration by a resin orpitch material, followed by pyrolyzation. The resulting carbon matrixcomposite, solid or porous, is then subjected to the electrochemicaldecomposition treatment to form the lithium- or sodium-containingspecies in the interstitial spaces, gaps, or voids (if any) but bondedto the pore walls, or simply bonded to surfaces of the carbon matrixcomposite to form a dendrite-intercepting layer. In such a dendritepenetration resistant layer, the graphene sheets or platelets orexfoliated graphite flakes are bonded by the carbon matrix and/or thelithium- or sodium-containing species.

For industrial-scale production of the presently invented dendritepenetration resistant layer (also referred to as a dendrite stopping ordendrite intercepting layer), the electrochemical decompositiontreatment may be carried out also in a roll-to-roll manner. In anembodiment, the continuous-length paper (or mat, foam, membrane, etc.)of carbon matrix, carbon/graphite reinforcement, or their carbon matrixcomposite may be unwound from a feeder roller, and moved to enter anelectrochemical treatment zone (essentially an electrochemicaldecomposition reactor) containing an electrolyte therein. A lithium orsodium electrode is immersed in this electrolyte and the paper is alsoelectrically wired as the working electrode. The paper is moved at acontrolled speed to give enough time for electrochemical decompositionof the electrolyte to occur. The paper, impregnated with and/or bondedby the decomposition products, is then wound up on a take-up roller.This roll-to-roll or reel-to-reel process can be easily scaled up andautomated for mass production of the presently invented dendritepenetration resistant layer products.

In an alternative embodiment, the continuous-length paper may be unwoundfrom a feeder roller, deposited with some lithium or sodium metal (e.g.using physical vapor deposition or sputtering of Li) while the paper isin a dry state (before contacting electrolyte). The Li- or Na-depositedpaper is then moved to enter an electrochemical treatment zonecontaining an electrolyte therein. As soon as the Li-paper layer orNa-paper layer enters the electrolyte, essentially short-circuitingoccurs between the carbonaceous/graphitic paper and Li (or Na). In otherwords, the paper “electrode” is essentially placed in an electrochemicalpotential that is 0 V with respect to Li⁺/Li or Na⁺/Na, subjecting theelectrolyte to a reductive decomposition and enabling decompositionproducts to react with graphene. Optionally, a lithium or sodiumelectrode is implemented and immersed in this electrolyte and thegraphene paper is also electrically wired as the working electrode. Suchan arrangement aids in continuing the electrochemical decomposition ofelectrolytes and formation of the bonding Li- or Na-containing species.The graphene paper is moved at a controlled speed to give enough timefor electrochemical decomposition of the electrolyte to occur. Thegraphene paper, impregnated with and bonded by the decompositionproducts, is then wound up on a take-up roller. Again, this roll-to-rollprocess is highly scalable and can be fully automated for cost-effectiveproduction of the desired dendrite-stopping layer product.

In yet another embodiment, a layer of alkali metal anode (e.g. astand-along Li foil or Na foil, or a nano-structured current collectordeposited with some lithium or sodium metal) is deposited with a layerof graphene sheets or exfoliated graphite flakes (e.g. using a sprayingprocedure) up to a thickness from 2 nm to 20 μm to form a two-layer orthree-layer laminate. Alternatively, a layer of alkali metal anode(containing one layer of Li/Na foil alone or a two-layer configurationcomposed of a nano-structured current collector and a layer of Li/Nametal, for instance) is directly laminated with a layer ofpre-fabricated graphene paper/mat to form a 2-layer or 3-layer laminate.This laminate is then combined with a porous separator and a cathode toform a multiple-layer battery structure, including an optional currentcollector, a Li or Na metal anode layer, a graphene or exfoliatedgraphite flake layer, a separator layer, a cathode active materiallayer, and an optional current collector layer. (The graphene orexfoliated graphite flake layer must be placed between the Li/Na metallayer and the separator layer.) This multiple-layer battery structure isthen made into a battery cell or multiple battery cells, which areimpregnated with a liquid electrolyte. As soon as the electrolyte isintroduced, the Li-graphene or Na-graphene pair is essentially in a“short-circuiting” condition. In other words, the graphene “electrode”is essentially placed in an electrochemical potential that is 0 V withrespect to Li⁺/Li or Na⁺/Na, subjecting the electrolyte to a reductivedecomposition and enabling decomposition products to react withgraphene. Optionally, the battery cell is subjected to acharge-discharge treatment to induce further oxidative and/or reductivedecomposition of the electrolyte (e.g. including at least a procedure ofcharging the battery cell to a voltage close to 4.7 V, say from 4.0 V to5.0 V, to induce oxidative decomposition of the electrolyte). Thedecomposition products chemically react with graphene sheets orexfoliated graphite flakes to form a dendrite-stopping layer in situinside a battery cell.

The dendrite-intercepting layer of the instant invention typicallyexhibits a lithium ion or sodium ion conductivity from 2.5×10⁻⁵ S/cm to5.5×10⁻³ S/cm, and more typically from 1.0×10⁻⁴ S/cm to 2.5×10⁻³ S/cm.There is no restriction on the thickness of the dendrite-interceptinglayer, but for practical purposes, the dendrite penetration-resistantlayer preferably has a thickness from 2 nm to 20 μm, more preferablyfrom 10 nm to 10 μm, and most preferably from 100 nm to 1 μm.

The cathode active material in this rechargeable alkali metal batterymay be selected from sulfur, selenium, tellurium, lithium sulfide,lithium selenide, lithium telluride, sodium sulfide, sodium selenide,sodium telluride, a chemically treated carbon or graphite materialhaving an expanded inter-graphene spacing d₀₀₂ of at least 0.4 nm, or anoxide, dichalcogenide, trichalcogenide, sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, vanadium, chromium, cobalt, manganese, iron, nickel, or acombination thereof. Preferred cathode active materials includenon-lithiated and slightly lithiated compounds having relatively highlithium or sodium storage capacities, such as TiS₂, MoS₂, MnO₂, CoO₂,and V₂O₅.

A novel family of 2D metal carbides or metal carbonides, now commonlyreferred to as MXenes, can be used as a cathode active material. MXenescan be produced by partially etching out certain elements from layeredstructures of metal carbides such as Ti₃AlC₂. For instance, an aqueous 1M NH₄HF₂ was used at room temperature as the etchant for Ti₃AlC₂.Typically, MXene surfaces are terminated by O, OH, and/or F groups,which is why they are usually referred to as M_(n+1)X_(n)T_(x), where Mis an early transition metal, X is C and/or N, T represents terminatinggroups (O, OH, and/or F), n=1, 2, or 3, and x is the number ofterminating groups. The MXene materials investigated include Ti₂CT_(x),(Ti_(0.5), Nb_(0.5))₂CT_(x), Nb₂CT_(x), V₂CT_(x), Ti₃C₂T_(x), (V_(0.5),Cr_(0.5))₃C₂T_(x), Ti₃CNT_(x), Ta₄C₃T_(x), and Nb₄C₃T_(x).

In an embodiment, the cathode layer contains an air cathode and thebattery is a lithium-air battery or sodium-air battery. In anotherembodiment, the cathode active material is selected from sulfur orlithium polysulfide and the battery is a lithium-sulfur or sodium-sulfurbattery. In yet another embodiment, the cathode active material may beselected from an organic or polymeric material capable of capturing orstoring lithium or sodium ions (e.g. via reversibly forming a redox pairwith lithium or sodium ion).

The electrolytic salts to be incorporated into an electrolyte of analkali metal secondary battery may be selected from a lithium salt suchas lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium [LiN(CF₃SO₂)₂], lithium bis(oxalato)borate(LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF3(CF₂CF₃)₃), lithiumbisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt, and theirsodium counterparts. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ arepreferred. The content of aforementioned electrolytic salts in thenon-aqueous solvent is preferably 0.5 to 3.0 M (mol/L).

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a Li or Na metal rechargeable battery.

At the anode side, lithium metal or sodium metal may be a layer of Li orNa metal or alloy (>70% by weight of Li or Na, preferably >80%, and morepreferably >90%) Alternatively, the Li or Na metal or alloy may besupported by a nano-structure composed of conductive nano-filaments. Forinstance, multiple conductive nano-filaments are processed to form anintegrated aggregate structure, preferably in the form of a closelypacked web, mat, or paper, characterized in that these filaments areintersected, overlapped, or somehow bonded (e.g., using a bindermaterial) to one another to form a network of electron-conducting paths.The integrated structure has substantially interconnected pores toaccommodate electrolyte. The nano-filament may be selected from, asexamples, a carbon nano fiber (CNF), graphite nano fiber (GNF), carbonnano-tube (CNT), metal nano wire (MNW), conductive nano-fibers obtainedby electro-spinning, conductive electro-spun composite nano-fibers,nano-scaled graphene platelet (NGP), or a combination thereof. Thenano-filaments may be bonded by a binder material selected from apolymer, coal tar pitch, petroleum pitch, meso-phase pitch, coke, or aderivative thereof.

Nano fibers may be selected from the group consisting of an electricallyconductive electro-spun polymer fiber, electro-spun polymernanocomposite fiber comprising a conductive filler, nano carbon fiberobtained from carbonization of an electro-spun polymer fiber,electro-spun pitch fiber, and combinations thereof. For instance, anano-structured electrode can be obtained by electro-spinning ofpolyacrylonitrile (PAN) into polymer nano-fibers, followed bycarbonization of PAN. It may be noted that some of the pores in thestructure, as carbonized, are greater than 100 nm and some smaller than100 nm.

In summary, a possible lithium metal cell may be comprised of an alkalimetal layer (e.g. Li foil, Na foil, etc.), an anode current collector(e.g. Cu foil and/or a nano-structure of interconnected conductivefilaments), a dendrite-intercepting layer, an electrolyte phase(typically supported by a porous separator, such as a porouspolyethylene-polypropylene co-polymer film), a cathode, and an optionalcathode current collector (e.g. Al foil and/or or a nano-structure ofinterconnected conductive filaments, such as graphene sheets and carbonnano-fibers). The dendrite-stopping layer is implemented between thealkali metal layer and the porous separator layer.

The following examples serve to illustrate the preferred embodiments ofthe present invention and should not be construed as limiting the scopeof the invention:

Example 1: Preparation of Graphene Oxide (GO) and Reduced Graphene Oxide(RGO) Nano Sheets from Natural Graphite Powder and their Paper/Mats(Layers of Porous Structure Prior to being Impregnated with AmorphousCarbon or Polymeric Carbon)

Natural graphite from Huadong Graphite Co. (Qingdao, China) was used asthe starting material. GO was obtained by following the well-knownmodified Hummers method, which involved two oxidation stages. In atypical procedure, the first oxidation was achieved in the followingconditions: 1100 mg of graphite was placed in a 1000 mL boiling flask.Then, 20 g of K₂S₂O₈, 20 g of P₂O₅, and 400 mL of a concentrated aqueoussolution of H₂SO₄ (96%) were added in the flask. The mixture was heatedunder reflux for 6 hours and then let without disturbing for 20 hours atroom temperature. Oxidized graphite was filtered and rinsed withabundant distilled water until neutral pH. A wet cake-like material wasrecovered at the end of this first oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

Surfactant-stabilized RGO (RGO-BS) was obtained by diluting the wet-cakein an aqueous solution of surfactants instead of pure water. Acommercially available mixture of cholate sodium (50 wt. %) anddeoxycholate sodium (50 wt. %) salts provided by Sigma Aldrich was used.The surfactant weight fraction was 0.5 wt. %. This fraction was keptconstant for all samples. Sonication was performed using a BransonSonifier S-250A equipped with a 13 mm step disruptor horn and a 3 mmtapered micro-tip, operating at a 20 kHz frequency. For instance, 10 mLof aqueous solutions containing 0.1 wt. % of GO was sonicated for 10 minand subsequently centrifuged at 2700 g for 30 min to remove anynon-dissolved large particles, aggregates, and impurities. Chemicalreduction of as-obtained GO to yield RGO was conducted by following themethod, which involved placing 10 mL of a 0.1 wt. % GO aqueous solutionin a boiling flask of 50 mL. Then, 10 μL of a 35 wt. % aqueous solutionof N₂H₄ (hydrazine) and 70 mL of a 28 wt. % of an aqueous solution ofNH₄OH (ammonia) were added to the mixture, which was stabilized bysurfactants. The solution was heated to 90° C. and refluxed for 1 h. ThepH value measured after the reaction was approximately 9. The color ofthe sample turned dark black during the reduction reaction.

These suspensions (GO in water and RGO in surfactant water) were thenfiltered through a vacuum-assisted membrane filtration apparatus toobtain porous GO and RGO paper or mat. The porous paper was infiltratedwith phenolic resin and carbonized at 350° C. for 2 hours, 550° C. for 2hours, and then 1,000° C. for another 2 hours to convert phenolic resininto polymeric carbon. The polymeric carbon infiltrated porous paper/matwas used as an electrode in an electrochemical decomposition reactor toform the dendrite-stopping layer. In some examples, the paper/mat wasalso used as a porous nano-structured electrode to support sulfur orlithium polysulfide and other active materials at the cathode.

Example 2: Preparation of Discrete Functionalized GO Sheets fromGraphite Fibers and Porous Films of Chemically Functionalized GO

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a pot of slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. Ammonia waterwas added to one pot of the resulting suspension, which wasultrasonicated for another hour to produce NH₂-functionalized grapheneoxide (f-GO). The GO sheets and functionalized GO sheets were separatelydiluted to a weight fraction of 5% and the suspensions were allowed tostay in the container without any mechanical disturbance for 2 days,forming liquid crystalline phase in the water-alcohol liquid whenalcohol is being vaporized at 80° C.

The resulting suspensions containing GO or f-GO liquid crystals werethen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. The resulting GO or f-GOcoating films, after removal of liquid, have a thickness that can bevaried from approximately 0.5 to 20 μm. Some of the resulting GO filmswere then subjected to heat treatments that involved an initial thermalreduction temperature of 80-350° C. for 2 hours, followed byheat-treating at a second temperature of 1,500-2,850° C. for differentspecimens to obtain various porous graphitic films. The porous film wasimpregnated with carbon using chemical vapor infiltration, which wasconducted by placing the porous film in a chamber and introducing amixture of acetylene and hydrogen gas into the chamber at 900° C. for1-4 hours. The CVI carbon-infiltrated film was used as an electrode inan electrochemical decomposition reactor to form the dendrite-stoppinglayer.

Example 3: Preparation of Single-Layer Graphene Sheets from Meso-CarbonMicro-Beads (MCMBs) and Fabrication of Carbon Matrix-Porous Graphene/CNTMats

Meso-carbon micro-beads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. In one example, MCMB (10grams) were intercalated with an acid solution (sulfuric acid, nitricacid, and potassium permanganate at a ratio of 4:1:0.05) for 48-96hours. Upon completion of the reaction, the mixture was poured intodeionized water and filtered. The intercalated MCMBs were repeatedlywashed in a 5% solution of HCl to remove most of the sulphate ions. Thesample was then washed repeatedly with deionized water until the pH ofthe filtrate was no less than 4.5. The slurry was then subjectedultrasonication for 10-100 minutes to fully exfoliate and separate GOsheets. TEM and atomic force microscopic studies indicate that most ofthe GO sheets were single-layer graphene when the oxidation treatmentexceeded 72 hours, and 2- or 3-layer graphene when the oxidation timewas from 48 to 72 hours. The GO sheets contain oxygen proportion ofapproximately 35%-47% by weight for oxidation treatment times of 48-96hours.

The suspension was then diluted to approximately 0.5% by weight in acontainer and approximately 0.5% by weight of multi-walled carbonnanotubes (CNTs) was added to this suspension to make a pot of slurry.The dispersion containing both single-layer graphene sheets and CNTs wasthen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. The resulting GO films, afterremoval of liquid, have a thickness from approximately 0.5 μm to 5 μm.The resulting GO/CNT compact was then subjected to heat treatments toproduce porous structures. These treatments typically involve an initialthermal reduction temperature of 80-500° C. for 1-3 hours, optionallyfollowed by heat-treating at a second temperature of 1,500° C. Theseporous films were then impregnated with a petroleum pitch at 250° C.,and then carbonized at 800° C. for 3 hours.

The resulting carbon matrix composite films, remaining porous, were usedas an electrode in an electrochemical decomposition reactor to form thedendrite-stopping layer. In some samples, these porous films were usedas a nano-structured current collector to support Li or Na metalthereon.

Example 4: Preparation of Pristine Graphene Sheets/Platelets (0% Oxygen)and the Effect of Pristine Graphene Sheets

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free) can lead to a HOGF having a higher thermalconductivity. Pristine graphene sheets were produced by using the directultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free. Thesuspension was then added with carbon black particles (CB-to-grapheneratio of 1/5) and then filtered via vacuum-assisted filtration to obtainporous paper structures. The porous paper, after infiltration withamorphous carbon, was used as an electrode in an electrochemicaldecomposition reactor to form the dendrite-stopping layer.

Example 5: Preparation of Polymeric Carbon by Pyrolyzation of a PolymerFilm

Several carbon films were produced by pyrolyzation of polyacrylonitrile(PAN) films. Solvent cast PAN films having an initial thickness fromapproximately 3 μm to 10 μm were made on a glass plate surface. Thesepolymer films were heat treated at an initial oxidation temperature of250° C. for 1 hour under a small biaxial tension stress. This wasfollowed by a carbonization treatment at 500° C. and then graduallyraised to 1,000° C. over a period of 4 hours in an argon atmosphere toproduce polymeric carbon films. Several polymeric carbon films were usedas an electrode in an electrochemical decomposition reactor to form thedendrite-stopping layer.

Example 6: Preparation of Amorphous Carbon on a Layer of Lithium Film

Lithium films, 1-5 μm thick, were deposited on surfaces of Cu foil orgraphene film (both serving as a current collector) using physical vapordeposition. The lithium film surface was in turn deposited with a layerof amorphous carbon, 0.5-3 μm thick. The resulting three-layer structurewas then covered with a porous separator, and a layer of cathode activematerial (coated on an Al foil current collector) to form a battery unitcell, which was then impregnated with a liquid electrolyte. Severalcathode active materials were used: LiCoO₂, V₂O₅, and graphene-supportedsulfur. The battery system was intentionally subjected to conditionsconducive to oxidative degradation of electrolyte (e.g. close to0.01-1.0 V vs. Li/Li⁺) and reductive degradation of electrolyte (e.g.4.1-4.9 V vs. Li/Li⁺) for the first 3 charge-discharge cycles to producethe desired lithium-containing species. After these, the battery isallowed to operate under normal operating conditions (e.g. cyclingbetween 1.5 and 3.8 volts for the Li-LiCoO₂ battery).

Example 7: Preparation of Amorphous Carbon on a Layer of Sodium Film

Sodium films, 1-5 μm thick, were deposited on surfaces of Cu foil orgraphene film (both serving as a current collector) using melt coating.The sodium film surface was in turn deposited with a layer of amorphouscarbon, 0.5-3 μm thick. The resulting three-layer structure was thencovered with a porous separator, and a layer of cathode active material(coated on an Al foil current collector) to form a battery unit cell,which was then impregnated with a liquid electrolyte. Several cathodeactive materials were used: Na_(x)V₂O₅ nano-belts from V₂O₅,MnO₂/graphene, and graphene supported sulfur. The battery system wasintentionally subjected to conditions conducive to oxidative degradationof electrolyte (e.g. close to 0.01-1.0 V vs. Na/Na⁺) and reductivedegradation of electrolyte (e.g. 3.8-4.6 V vs. Na/Na⁺) for the first 3charge-discharge cycles to produce the desired lithium-containingspecies. After these, the battery is allowed to operate under normaloperating conditions (e.g. cycling between 1.5 and 3.0 volts for theNa—S battery)

Example 8: Preparation of Fluorinated Graphite with ExpandedInter-Planar Spacing, Graphene Fluoride Nano Sheets, Porous GrapheneStructure from these Sheets, and Carbon-Infiltrated Graphene Structure

Several processes have been used by us to produce fluorinated graphiteparticles and, subsequently, graphene fluoride (GF) sheets, but only oneprocess is herein described as an example. In a typical procedure,intercalated compound C₂F.xClF₃ was further fluorinated by vapors ofchlorine trifluoride to yield fluorinated graphite (FG). Specifically,pre-cooled Teflon reactor was filled with 20-30 mL of liquid pre-cooledClF₃; the reactor was closed and cooled to liquid nitrogen temperature.Then, 0.5 g of lightly fluorinated graphite was put in a container withholes for ClF₃ gas to access and situated inside the reactor. In 7-10days a gray-beige product with approximate formula C₂F was formed.Depending upon the reaction time, the inter-planar spacing (d₀₀₂, asmeasured by X-ray diffraction) was varied from approximately 0.55 nm to0.97 nm. Portion of these graphite fluoride particles was used as a zincion intercalation compound due to their expanded inter-planar spacesbeing surprisingly conducive to entry by zinc ions.

Subsequently, a desired amount of graphite fluoride (approximately 0.5g) was mixed with 20 L of an organic solvent (methanol, ethanol,1-propanol, 2-propanol, or 1-butanol) and subjected to an ultrasoundtreatment (280 W) for 30 min, leading to the formation of homogeneousyellowish dispersions. Five minutes of sonication was enough to obtain arelatively homogenous dispersion of few-layer graphene fluoride, butlonger sonication times ensured the production of mostly single-layergraphene fluoride sheets. Some of these suspension samples weresubjected to vacuum oven drying to recover separated graphene fluoridesheets. These graphene fluoride sheets were then added into apolymer-solvent or monomer-solvent solution to form a suspension.Various polymers (e.g. polyethylene oxide) or monomers (e.g.caprolactam) were utilized as the precursor film materials forsubsequent carbonization and graphitization treatments.

Upon casting on a glass surface with the solvent removed, the dispersionbecame a brownish film formed on the glass surface. When theseGF-reinforced polymer films were heat-treated, some fluorine and othernon-carbon elements were released as gases that generated pores in thefilm. The resulting porous graphitic films had physical densities from0.33 to 1.22 g/cm³. These porous graphitic films were then roll-pressedto obtain graphitic films (porous structures) having a density from 0.8to 1.5 g/cm³. These porous structures were infiltrated with either CVDcarbon or polymeric carbon and were used as an electrode in anelectrochemical decomposition reactor to form the dendrite-stoppinglayer.

Example 9: Preparation of Nitrogenataed Graphene Nano Sheets, PorousGraphene Structures, and their Carbon Matrix Composites

Graphene oxide (GO), synthesized in Example 1, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene: urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenataed graphene sheets remaindispersible in water. A zinc ion intercalation compounds (MnO₂ and MoS₂,respectively) was added into the nitrogenataed graphene-water suspensionto form a pot of slurry. The resulting slurries were then cast and driedto produce porous graphene structures, which were infiltrated with CVIcarbon and used as an electrode in an electrochemical decompositionreactor to form the dendrite-stopping layer.

Example 10: Exfoliated Graphite Worms from Natural Graphite, and theirCarbon Matrix Composites

Natural graphite, nominally sized at 45 μm, provided by Asbury Carbons(405 Old Main St., Asbury, N.J. 08802, USA) was milled to reduce thesize to approximately 14 μm. The chemicals used in the present study,including fuming nitric acid (>90%), sulfuric acid (95-98%), potassiumchlorate (98%), and hydrochloric acid (37%), were purchased fromSigma-Aldrich and used as received.

A reaction flask containing a magnetic stir bar was charged withsulfuric acid (360 mL) and nitric acid (180 mL) and cooled by immersionin an ice bath. The acid mixture was stirred and allowed to cool for 15min, and graphite (20 g) was added under vigorous stirring to avoidagglomeration. After the graphite powder was well dispersed, potassiumchlorate (110 g) was added slowly over 15 min to avoid sudden increasesin temperature. The reaction flask was loosely capped to allow evolutionof gas from the reaction mixture, which was stirred for 48 hours at roomtemperature. On completion of the reaction, the mixture was poured into8 L of deionized water and filtered. The slurry was spray-dried torecover an expandable graphite sample. The dried, expandable graphitewas quickly placed in a tube furnace preheated to 1,000° C. and allowedto stay inside a quartz tube for approximately 40 seconds to obtainexfoliated graphite worms. Some of the graphite worms were thenroll-pressed to obtain samples of re-compressed exfoliated graphitehaving a range of physical densities (e.g. 0.3 to 1.2 g/cm³). Theseporous, re-compressed structures were infiltrated with CVI carbon andused as an electrode in an electrochemical decomposition reactor to formthe dendrite-stopping layer.

Example 11: Exfoliated Graphite Worms from Various Synthetic GraphiteParticles or Fibers, and their Carbon Matrix Composites

Additional exfoliated graphite worms were prepared according to the sameprocedure described in Example 1, but the starting graphite materialswere graphite fiber (Amoco P-100 graphitized carbon fiber), graphiticcarbon nano-fiber (Pyrograph-III from Applied Science, Inc., Cedarville,Ohio), spheroidal graphite (HuaDong Graphite, QinDao, China), andmeso-carbon micro-beads (MCMBs) (China Steel Chemical Co., Taiwan),respectively. These four types of laminar graphite materials wereintercalated and exfoliated under similar conditions as used for Example1 for different periods of time, from 24 hours to 96 hours.

Some amount of the graphite forms was then roll-pressed to obtainsamples of re-compressed exfoliated graphite having a range of physicaldensities (e.g. 0.3 to 1.2 g/cm³). A second amount of the graphite wormswas subjected to low-intensity sonication to produce expanded graphiteflakes. These expanded graphite flakes were then made into a paper form(the porous structure) using the vacuum-assisted filtration technique.The porous paper was impregnated with polymeric carbon and used as anelectrode in an electrochemical decomposition reactor to form thedendrite-stopping layer.

Example 12: Exfoliated Graphite Worms from Natural Graphite UsingHummers Method and their Carbon Matrix Composites

Additional graphite intercalation compound (GIC) was prepared byintercalation and oxidation of natural graphite flakes (original size of200 mesh, from Huadong Graphite Co., Pingdu, China, milled toapproximately 15 μm) with sulfuric acid, sodium nitrate, and potassiumpermanganate according to the method of Hummers [U.S. Pat. No.2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite,we used a mixture of 22 ml of concentrated sulfuric acid, 2.8 grams ofpotassium permanganate, and 0.5 grams of sodium nitrate. The graphiteflakes were immersed in the mixture solution and the reaction time wasapproximately three hours at 30° C. It is important to caution thatpotassium permanganate should be gradually added to sulfuric acid in awell-controlled manner to avoid overheat and other safety issues. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The sample was then washed repeatedly with deionized wateruntil the pH of the filtrate was approximately 5. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theresulting GIC was exposed to a temperature of 1,050° C. for 35 secondsin a quartz tube filled with nitrogen gas to obtain worms of exfoliatedgraphite flakes.

Some of the graphite forms were then roll-pressed to obtain samples ofre-compressed exfoliated graphite having a range of physical densities(e.g. 0.3 to 1.2 g/cm³). Some of the graphite worms were subjected tolow-intensity sonication to produce expanded graphite flakes. Theseexpanded graphite flakes were then made into a porous paper form usingthe vacuum-assisted filtration technique. The porous paper wasimpregnated with polymeric carbon (petroleum pitch and then carbonizedat 900° C.). The resulting carbon matrix composites were used as anelectrode in an electrochemical decomposition reactor to form thedendrite-stopping layer.

Example 13: Electrochemical Preparation of Dendrite-Intercepting LayersContaining Carbon Matrix or Carbon Matrix Composite Bonded by LithiumChemical Species

The preparation of dendrite-stopping layers was carried out in anelectrochemical reactor, an apparatus very similar to an electrodeplating system. In this reactor, a layer of carbon matrix or carbonmatrix composite structure (in the form of a mat, paper, film, etc. asprepared in previous 12 examples) was used as a working electrode andlithium sheet as both the counter and reference electrodes. Inside thereactor is an electrolyte composed of 1M LiPF₆ dissolved in a mixture ofethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume), asan example. The carbon matrix or composite layer in the workingelectrode was galvanostatically discharged (Li ions being sent to thisworking electrode) and charged (Li ions partially released by thisworking electrode) in the voltage range from 0.01V to 4.9V at thecurrent densities of 100-1000 mA/g following a voltage-current programsimilar to what would be used in a lithium-ion battery. However, thesystem was intentionally subjected to conditions conducive to oxidativedegradation of electrolyte (close to 0.01-1.0 V vs. Li/Li⁺) or reductivedegradation of electrolyte (e.g. 4.1-4.9 V vs. Li/Li⁺) for a sufficientlength of time. The degradation products react with Li⁺ ions, Li salt,functional groups (if any) or carbon atoms on the carbon matrix orcomposite to form the lithium-containing species that are alsochemically bonded to the carbon matrix and/or carbon/graphiticreinforcement materials (e.g. CNTs, CNFs, graphene, exfoliated graphiteflakes, etc.) dispersed therein.

The chemical compositions of the lithium-containing species are governedby the voltage range, the number of cycles (from 0.01 V to 4.9 V, andback), solvent type, lithium salt type, chemical composition of carbonmatrix and the carbon/graphite reinforcement phase (e.g. % of O, H, andN), and electrolyte additives (e.g. LiNO₃), if available.

The morphology, structure and composition of the carbon matrix,composites, the lithium-containing species that are bonded to carbonmatrix or composites were characterized by scanning electron microscope(SEM), transmission electron microscope (TEM), Raman spectrum, X-raydiffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Anextensive investigation that covers a broad range of lithium salts,solvents, and additives lead to the following discoveries:

A wide range of lithium-containing species were formed in a controlledmanner and these species were well-bonded to the carbon matrix and/orcarbon/graphite reinforcement phase. The resulting lithium chemicalspecies-bonded integral layers are of structural integrity, robustenough to intercept metal dendrites or stop dendrite penetration throughthese layers. It may be noted that, quite surprisingly, the layerscontaining lithium chemical species—bonded structures are not only goodfor stopping lithium dendrite in lithium metal batteries, but alsoeffective in intercepting sodium dendrite in sodium metal batteries.These layers are electrochemically very stable in the sodium saltenvironment as well.

In these layers, species (CH₂OCO₂Li)₂ is a two-electron reductionproduct of ethylene carbonate (EC) in an EC-based electrolytes. ROCO₂Lispecies are present between carbon or graphitic material in electrolytescontaining propylene carbonate (PC), especially when the concentrationof PC in the electrolyte is high. Li₂CO₃ is present on carbon matrix orcarbon/graphite reinforcement surfaces in EC or PC based electrolyteswhen a higher voltage is imposed for an extended period of time. Thisspecies also appear as a reaction product of semi-carbonates with HF orwater or CO₂. ROLi is produced on carbon matrix in ether electrolytessuch as tetrahydrofuran (THF), dimethyl carbonate (DMC), or ethyl methylcarbonate (EMC) as an electrochemical reduction product at anelectrochemical potential lower than 0.5 V vs. Li/Li⁺. LiF is readilyformed in electrolytes containing fluorinated salts such as LiAsF₆,LiPF₆, and LiBF₄, as a salt reduction product. Li₂O is a degradationproduct of Li₂CO₃. LiOH is formed when a small but controlled amount ofwater is added to the reactor. Species such as Li₂C₂O₄, Li carboxylate,Li methoxide, are formed in electrolytes containing 1-2 M of LiPF₆ inEC:EMC (e.g. at a 3:7 ratio). HCOLi is formed when methyl formate isused as a co-solvent or additive in the electrolyte.

Example 14: Electrochemical Preparation of Dendrite-Intercepting LayersContaining Carbon Matrix or Carbon Matrix Composite Bonded by/toSodium-Containing Chemical Species

The same electrochemical reactor was used for preparation ofdendrite-stopping layers featuring sodium-containing chemical species.Again, a carbon matrix or carbon matrix composite structure (in the formof a mat, paper, film, etc.) was used as a working electrode and sodiumsheet or rod as both the counter and reference electrodes. Inside thereactor is an electrolyte composed of a sodium salt dissolved in asolvent or mixture of solvents.

Sodium salts used in this example include sodium perchlorate (NaClO₄),sodium hexafluorophosphate (NaPF₆), sodium borofluoride (NaBF₄), andsodium trifluoromethanesulfonimide (NaTFSI). Solvents used include1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), methyl acetate (MA), fluoroethylene carbonate (FEC),vinylene carbonate (VC), and allyl ethyl carbonate (AEC).

The carbon matrix or carbon matrix composite materials in the workingelectrode were galvanostatically discharged (Na ions being sent to thecarbon matrix or carbon matrix composite) and charged (Na ions releasedby carbon matrix or carbon matrix composite) in the voltage range from0.01V to 4.7V at the current densities of 100-1000 mA/g following avoltage-current program similar to what would be used in a lithium-ionbattery. However, the system was intentionally subjected to conditionsconducive to oxidative degradation of electrolyte (close to 0.01 V-0.8 Vvs. Na/Na⁺) or reductive degradation of electrolyte (3.8-4.7 V vs.Na/Na⁺) for a sufficient length of time. The degradation products reactwith Na⁺ ions, Na salt, functional groups (containing O, H, N, etc.) orcarbon atoms on carbon matrix or carbon matrix composite to form thelithium-containing species that are also chemically bonded to the carbonmatrix and/or carbon/graphite reinforcement phase.

The chemical compositions of the sodium-containing species are dictatedby the voltage range, the number of cycles (from 0.01 V to 4.7 V, andback), solvent type, sodium salt type, chemical composition of graphenesheets (e.g. % of O, H, and N), and electrolyte additives (e.g. NaNO₃),if available. An extensive investigation that covers a broad range oflithium salts, solvents, and additives lead to the followingdiscoveries:

A wide range of sodium-containing species were formed in a controlledmanner and these species are typically well-bonded to the carbon matrixor carbon matrix composite. The resulting integral layers are ofstructural integrity, strong enough to intercept metal dendrites or stopdendrite penetration through these layers.

In these layers, species (CH₂OCO₂Na)₂ is believed to be a two-electronreduction product of ethylene carbonate (EC) in an EC-basedelectrolytes. ROCO₂Na species are present on carbon matrix or carbonmatrix composite in electrolytes containing propylene carbonate (PC),especially when the concentration of PC in the electrolyte is high.Na₂CO₃ is present on carbon matrix or carbon matrix composite in EC orPC based electrolytes when a higher voltage is imposed for an extendedperiod of time. This species also appear as a reaction product ofsemi-carbonates with HF or water or CO₂.

RONa is produced on carbon matrix or carbon matrix composite in etherelectrolytes such as tetrahydrofuran (THF), dimethyl carbonate (DMC), orethyl methyl carbonate (EMC) as an electrochemical reduction product atan electrochemical potential lower than 0.5 V vs. Na/Na⁺. NaF is readilyformed in electrolytes containing fluorinated salts such as NaAsF₆,NaPF₆, and NaBF₄, as a salt reduction product. Na₂O is a degradationproduct of Na₂CO₃. NaOH is formed when a small but controlled amount ofwater is added to the reactor. Species such as Na₂C₂O₄, Na carboxylate,Na methoxide, are formed in electrolytes containing 1-2 M of NaPF₆ inEC:EMC (e.g. at a 3:7 ratio). HCONa is formed when methyl formate isused as a co-solvent or additive in the electrolyte.

Example 15: Preparation of MoS₂/RGO Hybrid Cathode Material for Li Metaland Na Metal Cells and MOS₂ as a Filler Dispersed in a Carbon Matrix

Ultra-thin MoS₂/RGO hybrid was synthesized by a one-step solvothermalreaction of (NH₄)₂MOS₄ and hydrazine in an N, N-dimethylformamide (DMF)solution of oxidized graphene oxide (GO) at 200° C. In a typicalprocedure, 22 mg of (NH₄)₂MOS₄ was added to 10 mg of GO dispersed in 10ml of DMF. The mixture was sonicated at room temperature forapproximately 10 min until a clear and homogeneous solution wasobtained. After that, 0.1 ml of N₂H₄•H₂O was added. The reactionsolution was further sonicated for 30 min before being transferred to a40 mL Teflon-lined autoclave. The system was heated in an oven at 200°C. for 10 h. Product was collected by centrifugation at 8000 rpm for 5min, washed with DI water and recollected by centrifugation. The washingstep was repeated for at least 5 times to ensure that most DMF wasremoved. Finally, product was dried and made into a cathode. On aseparate basis, several different amounts (5% to 45% by weight) of MoS₂platelets were added into a carbon matrix to form a composite sheet. Thestrength and surface hardness of the resulting composite were found tobe significantly higher than those of the corresponding carbon matrixalone.

Example 16: Preparation of Two-Dimensional (2D) Layered Bi₂Se₃Chalcogenide Nanoribbons for Both Li Metal and Na Metal Cells (as aCathode Active Material and/or and as a Filler Dispersed in a CarbonMatrix)

The preparation of (2D layered Bi₂Se₃ chalcogenide nanoribbons iswell-known in the art. For instance, Bi₂Se₃ nanoribbons were grown usingthe vapor-liquid-solid (VLS) method. Nanoribbons herein produced are, onaverage, 30-55 nm thick with widths and lengths ranging from hundreds ofnanometers to several micrometers. Larger nanoribbons were subjected toball-milling for reducing the lateral dimensions (length and width) tobelow 200 nm. Nanoribbons prepared by these procedures (with or withoutthe presence of graphene sheets or exfoliated graphite flakes) werefound to have sharp edges and high crystallinity, which enable maximumintercalation by Li or Na ions. The Bi₂Se₃ nanoribbons were used toreinforce the carbon matrix for improved penetration resistance ofdendrites.

Example 17: MXenes as Examples of a Cathode Active Material for Both LiMetal and Na Metal Cells, and/or as a Filler Dispersed in a CarbonMatrix

Selected MXenes, were produced by partially etching out certain elementsfrom layered structures of metal carbides such as Ti₃AlC₂. For instance,an aqueous 1 M NH₄HF₂ was used at room temperature as the etchant forTi₃AlC₂. Typically, MXene surfaces are terminated by O, OH, and/or Fgroups, which is why they are usually referred to as M_(n+1)X_(n)T_(x),where M is an early transition metal, X is C and/or N, T representsterminating groups (O, OH, and/or F), n=1, 2, or 3, and x is the numberof terminating groups. The MXene materials investigated includeTi₂CT_(x), Nb₂CT_(x), V₂CT_(x), Ti₃CNT_(x), and Ta₄C₃T_(x). Typically,85% MXene, 8% graphene, and 7% PVDF binder were mixed and made into acathode layer on an Al foil current collector.

Example 18: Preparation of Various MnO₂—Graphene Cathodes for Both LiMetal and Na Metal Cells

The MnO₂ powder was synthesized by two methods (each with or without thepresence of graphene sheets). In one method, a 0.1 mol/L KMnO₄ aqueoussolution was prepared by dissolving potassium permanganate in deionizedwater. Meanwhile 13.32 g surfactant of high purity sodiumbis(2-ethylhexyl) sulfosuccinate was added in 300 mL iso-octane (oil)and stirred well to get an optically transparent solution. Then, 32.4 mLof 0.1 mol/L KMnO₄ solution was added in the solution, which wasultrasonicated for 30 min to prepare a dark brown precipitate. Theproduct was separated, washed several times with distilled water andethanol, and dried at 80° C. for 12 h. The sample is MnO₂. The batteriesare each composed of a graphene/MnO₂-based cathode, a Li or Nametal-based anode.

Example 19: Preparation of Graphene-Enabled Li_(x)V₃O₈ Nano-Sheets fromV₂O₅ and LiOH as a Cathode for Li Metal Cell

All chemicals used in this study were analytical grade and were used asreceived without further purification. V₂O₅ (99.6%, Alfa Aesar) and LiOH(99+%, Sigma-Aldrich) were used to prepare the precursor solution.Graphene oxide (GO, 1% w/v obtained in Example 2 above) was used as astructure modifier. First, V₂O₅ and LiOH in a stoichiometric V/Li ratioof 1:3 were dissolved in actively stirred de-ionized water at 50° C.until an aqueous solution of Li_(x)V₃O₈ was formed. Then, GO suspensionwas added while stirring, and the resulting suspension was atomized anddried in an oven at 160° C. to produce the spherical compositeparticulates of GO/Li_(x)V₃O₈ nano-sheets and the sample was designatedNLVO-1. Corresponding Li_(x)V₃O₈ materials were obtained undercomparable processing conditions, but without graphene oxide sheets. Thesample was designated as LVO-2.

The Nyquist plots obtained from electrical impedance tests show asemicircle in the high to medium frequency range, which describes thecharge-transfer resistance for both electrodes. The intercept value isconsidered to represent the total electrical resistance offered by theelectrolyte. The inclined line represents the Warburg impedance at lowfrequency, which indicates the diffusion of ions in the solid matrix.The values of Rct for the vanadium oxide alone and graphene-enhancedcomposite electrodes are about 50.0 and 350.0 Ω for NLVO-1 and LVO-2,respectively. The Rct of the composite electrode is much smaller thanthat of the LVO electrode. Therefore, the presence of graphene (<2% byweight in this case) in the vanadium oxide composite has dramaticallyreduced the internal charge transfer resistance and improved the batteryperformance upon extended cycling.

An additional set of graphene-enabled Li_(x)V₃O₈ nano-sheet compositeparticulates was produced from V₂O₅ and LiOH under comparableconditions, but was dried under different atomization temperatures,pressures, and gas flow rates to achieve four samples of compositeparticulates with four different Li_(x)V₃O₈ nano-sheet averagethicknesses (4.6 nm, 8.5 nm, 14 nm, and 35 nm). A sample of Li_(x)V₃O₈sheets/rods with an average thickness/diameter of 76 nm was alsoobtained without the presence of graphene oxide sheets (but, with thepresence of carbon black particles) under the same processing conditionsfor the graphene-enhanced particulates with a nano-sheet averagethickness of 35 nm. It seems that carbon black is not as good anucleating agent as graphene for the formation of Li_(x)V₃O₈ nano-sheetcrystals. The specific capacity of these cathode materials wasinvestigated using Li foil as a counter electrode.

Example 20: Hydrothermal Synthesis of Graphene—Enabled Na_(x)V₂O₅Nano-belts from V₂O₅, NaCl, and Graphene Oxide for Na Metal Cells

In a typical experiment, vanadium pentoxide gels were obtained by mixingV₂O₅ in a NaCl aqueous solution. The Na⁺-exchanged gels obtained byinteraction with NaCl solution (the Na:V molar ratio was kept as 1:1),with or without mixing by a GO suspension, were placed in a Teflon-linedstainless steel 35 ml autoclave, sealed, and heated up to 180° C. for8-24 h. After such a hydrothermal treatment, the green solids werecollected, thoroughly washed, optionally ultrasonicated, and dried at70° C. for 12 h followed by either (a) drying at 200° C. under vacuumovernight to obtain paper-like lamella composite structures or (b)mixing with another 0.1% GO in water, ultrasonicating to break downnano-belt sizes, and then spray-drying at 200° C. to obtaingraphene-embraced composite particulates.

Example 21: Electrochemical Deposition of S on Various Webs or Paper forLi—S And Room Temperature Na—S Batteries

The electrochemical deposition of sulfur (S) was conducted before thecathode active layer was incorporated into an alkali metal-sulfurbattery cell (Li—S and Na—S). The anode, the electrolyte, and theintegral layer of porous graphene structure (serving as a cathode layer)are positioned in an external container outside of a lithium-sulfurcell. The needed apparatus is similar to an electro-plating system,which is well-known in the art.

In a typical procedure, a metal polysulfide (Li₂S₉ and Na₂S) isdissolved in a solvent (e.g. mixture of DOL/DME at a volume ratio from1:3 to 3:1) to form an electrolyte solution. The electrolyte solution isthen poured into a chamber or reactor under a dry and controlledatmosphere condition (e.g. He or Nitrogen gas). A metal foil was used asthe anode and a layer of the porous graphene structure as the cathode;both being immersed in the electrolyte solution. This configurationconstitutes an electrochemical deposition system. The step ofelectrochemically depositing nano-scaled sulfur particles or coating onthe graphene surfaces was conducted at a current density preferably inthe range of 1 mA/g to 10 A/g, based on the layer weight of the porousgraphene structure.

The chemical reactions that occurred in this reactor may be representedby the following equation: M_(x)S_(y)→M_(x)S_(y-z)+zS (typically z=1-4).Quite surprisingly, the precipitated S is preferentially nucleated andgrown on massive graphene surfaces to form nano-scaled coating or nanoparticles. The coating thickness or particle diameter and the amount ofS coating/particles was controlled by the specific surface area,electro-chemical reaction current density, temperature and time. Ingeneral, a lower current density and lower reaction temperature lead toa more uniform distribution of S and the reactions are easier tocontrol. A longer reaction time leads to a larger amount of S depositedon graphene surfaces and the reaction is ceased when the sulfur sourceis consumed or when a desired amount of S is deposited.

The specific discharge capacities of three Li—S cells (one containingthe presently invented carbon matrix based dendrite-intercepting layer,one containing graphene-based dendrite-intercepting layer, and onecontaining no dendrite-stopping layer) were plotted as a function of thenumber of cycles (FIG. 7). Quite unexpectedly, the cells containing adendrite-intercepting layer actually show a more stable cycling behavioreven though all three cells have otherwise identical configurations. Itappears that the invented dendrite-intercepting layer not only serves tostop dendrite penetration but also block the lithium polysulfide speciesthat were dissolved in the liquid electrolyte and migrated from thecathode toward the anode side. Such a blocking action prevents thelithium polysulfide from reaching and reacting with the Li metal at theanode, which otherwise would form Li₂S that irreversibly deposits on Limetal surface. This so-called shuttling effect in all Li—S cells isactually significantly reduced due to the presence of thisdendrite-intercepting layer. This is an un-intended, but highlybeneficial result.

We have achieved a specific energy >800 Wh/kg in many Li—S cellsand >600 Wh/kg in room-temperature Na—S cells. None of these cells haveany dendrite issue.

Example 22: Li—S and Room Temperature Na—S Cells Containing ChemicalReaction Deposited Sulfur Particles as the Cathode

A chemical deposition method is herein utilized to prepare S-graphenecomposites from isolated graphene oxide sheets (i.e. these GO sheetswere not packed into an integral structure of porous graphene prior tochemical deposition of S on surfaces of GO sheets). The procedure beganwith adding 0.58 g Na₂S into a flask that had been filled with 25 mldistilled water to form a Na₂S solution. Then, 0.72 g elemental S wassuspended in the Na₂S solution and stirred with a magnetic stirrer forabout 2 hours at room temperature. The color of the solution changedslowly to orange-yellow as the sulfur dissolved. After dissolution ofthe sulfur, a sodium polysulfide (Na₂S_(x)) solution was obtained(x=4−10).

Subsequently, a graphene oxide-sulfur (GO—S) composite was prepared by achemical deposition method in an aqueous solution. First, 180 mg ofgraphite oxide was suspended in 180 ml ultrapure water and thensonicated at 50° C. for 5 hours to form a stable graphene oxide (GO)dispersion. Subsequently, the Na₂S_(x) solution was added to theabove-prepared GO dispersions in the presence of 5 wt. % surfactantcetyl trimethyl-ammonium bromide (CTAB), the as-prepared GO/Na₂S,blended solution was sonicated for another 2 hours and then titratedinto 100 ml of 2 mol/L HCOOH solution at a rate of 30-40 drops/min andstirred for 2 hours. Finally, the precipitate was filtered and washedwith acetone and distilled water several times to eliminate salts andimpurities. After filtration, the precipitate was dried at 50° C. in adrying oven for 48 hours. The reaction may be represented by thefollowing reaction: S_(x) ²⁻+2H⁺→(x−1) S+H₂S. The GO—S composite wasused as a cathode active material in a Li—S cell and room temperatureNa—S cell.

Example 23: Li-Air and Na-Air Cells with Ionic Liquid Electrolytes

To test the performance of the Li-air and Na-air batteries employing adendrite-intercepting layer and those without such a layer, severalpouch cells with dimension of 5 cm×5 cm were built. Porous carbonelectrodes were prepared by first preparing ink slurries by dissolving a90 wt. % EC600JD Ketjen black (AkzoNobel) and 5 wt. % Kynar PVDF (ArkemaCorporation) in Nmethyl-2-pyrrolidone (NMP). Air electrodes wereprepared with a carbon loading of approximately 20.0 mg/cm² byhand-painting the inks onto a carbon cloth (PANEX 35, ZoltekCorporation), which was then dried at 180° C. overnight. The totalgeometric area of the electrodes was 3.93 cm². The Li/O₂ and Na/O₂ testpouch cells were assembled in an argon-filled glove box. Each cellconsists of metallic lithium or sodium anode and the air electrode as acathode, prepared as mentioned above. A copper current collector for theanode and an aluminum current collector for the cathode were used. ACelgard 3401 separator separating the two electrodes was soaked in 1 MLiTFSI-DOL/EMITFSI (6/4) solutions for a minimum of 24 hours. Thecathode was soaked in the oxygen saturated EMITFSI-DOL/LiTF SI solutionfor 24 hours and was placed under vacuum for an hour before beingincluded in the cell assembly.

The cells were placed in an oxygen-filled glove box where oxygenpressure was maintained at 1 atm. Cell charge-discharge was carried outwith a battery cycler at the current rate of 0.1 mA/cm² at roomtemperature. It was found surprisingly that a Li-air or Na-air featuringa dendrite-intercepting layer exhibits a higher round-trip efficiencyfor cells (75% and 68%) as compared with their counterparts without sucha dendrite-stopping layer (64% and 55%, respectively, for the Li-air andNa-air cell). Most significantly, the cells without such adendrite-stopping layer tended to fail in 25-35 charge-discharge cycles.In contrast, the presently invented cells containing such adendrite-stopping layer usually lasted for more than 100 cycles withoutexperiencing any dendrite-induced failure.

Examples 24: Evaluation of Electrochemical Performance of Various Li andNa Secondary Metal Batteries

A broad array of Li metal and Na metal secondary (rechargeable)batteries were investigated. None of the batteries containing adendrite-intercepting layer prepared according to instant invention werefound to fail due to dendrite penetration through the separator layer.In fact, no dendrite was found to penetrate the inventeddendrite-intercepting layer based on the observation of post-testinginspection on a large number of battery cells.

Li or Na ion storage capacities of many cells were measured periodicallyand recorded as a function of the number of cycles. The specificdischarge capacity herein referred to is the total charge inserted intothe cathode during the discharge, per unit mass of the composite cathode(counting the weights of cathode active material, conductive additive orsupport, binder, and any optional additive combined). The specificcharge capacity refers to the amount of charges per unit mass of thecomposite cathode. The specific energy and specific power valuespresented in this section are based on the total cell weight. Themorphological or micro-structural changes of selected samples after adesired number of repeated charging and recharging cycles were observedusing both transmission electron microscopy (TEM) and scanning electronmicroscopy (SEM). The presently invented dendrite-intercepting layerenables the safe operation of many lithium metal and sodium metalsecondary batteries capable of storing an energy density of 300-400Wh/kg (e.g. lithium metal-metal oxide cells and Na—S cells), 400-900Wh/kg (e.g. Li—S cells), and >1,000 Wh/kg (e.g. Li-air cells) for a longcycle life without a dendrite penetration problem.

In summary, the present invention provides an innovative, versatile, andsurprisingly effective platform materials technology that enables thedesign and manufacture of superior and safe alkali metal and sodiummetal rechargeable batteries. The lithium dendrite or sodium dendriteissue in these high energy and high power cells is essentiallyeliminated, making it possible for these batteries to be widelyimplemented for electric vehicle, renewable energy storage, andelectronic device applications.

The invention claimed is:
 1. A dendrite penetration-resistant layer fora rechargeable alkali metal battery, said layer comprising an amorphouscarbon matrix, a carbon-containing reinforcement phase dispersed in saidmatrix, and a sodium-containing species that is chemically bonded tosaid matrix to form an integral layer that prevents dendrite penetrationthrough said integral layer in said alkali metal battery, wherein thesodium-containing species is selected from Na₂CO₃, Na₂O, Na₂C₂O₄, NaOH,NaX, ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂, (CH₂OCO₂Na)₂, Na₂S, Na_(x)SO_(y),or combinations thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbongroup, 0<x≤1 and 1≤y≤4; and wherein the sodium-containing species isderived from an electrochemical decomposition reaction, wherein thecarbon matrix is from 5% to 95% by volume of the integral layer, andwherein a weight ratio of the carbon matrix to the sodium-containingspecies is from 1/100 to 100/1.
 2. The dendrite penetration-resistantlayer of claim 1, wherein the carbon-containing reinforcement phasefurther comprises a material selected from soft carbon particles, hardcarbon particles, expanded graphite flakes, carbon black particles,carbon nanotubes, carbon nanofibers, carbon fibers, graphite fibers,polymer fibers, coke particles, mesophase carbon particles, mesoporouscarbon particles, electro-spun conductive nanofibers, carbon-coatedmetal nanowires, conductive polymer-coated nanowires or nanofibers,graphene sheets, graphene platelets, or combinations thereof.
 3. Thedendrite penetration-resistant layer of claim 2, wherein said graphenesheets or graphene platelets include single-layer sheets or multi-layerplatelets of a graphene material selected from graphene with essentially0% oxygen, graphene oxide having 2% to 46% by weight of oxygen, reducedgraphene oxide having 0.01% to 2% by weight of oxygen, chemicallyfunctionalized graphene, nitrogen-doped graphene, boron-doped graphene,fluorinated graphene, and combinations thereof.
 4. The dendritepenetration-resistant layer of claim 2 wherein the graphene sheets orplatelets contain single-layer or few-layer graphene, wherein few-layeris defined as 10 planes of hexagonal carbon atoms or less.
 5. Thedendrite penetration-resistant layer of claim 1, wherein the amorphousmatrix is obtained by sputtering of carbon, chemical vapor deposition,or chemical vapor infiltration.
 6. The dendrite penetration-resistantlayer of claim 1, wherein the amorphous carbon further containsparticles of a transition metal oxide or transition metal sulfide. 7.The dendrite penetration-resistant layer of claim 6, wherein theparticles of a transition metal oxide or transition metal sulfide areselected from an oxide or sulfide of niobium, zirconium, molybdenum,hafnium, tantalum, tungsten, titanium, vanadium, chromium, cobalt,manganese, iron, or nickel in a nanowire, nanodisc, nanoribbon, ornanoplatelet form.
 8. The dendrite penetration-resistant layer of claim1, wherein the dendrite penetration-resistant layer has a thickness from10 nm to 20 μm.
 9. The dendrite penetration-resistant layer of claim 1,wherein the dendrite penetration-resistant layer has a thickness from100 nm to 10 μm.
 10. The dendrite penetration-resistant layer of claim1, wherein the dendrite penetration-resistant layer is a sodium ionconductor having an ion conductivity from 2.5×10⁻⁵ S/cm to 5.5×10⁻³S/cm.
 11. The dendrite penetration-resistant layer of claim 1, whereinthe carbon-containing reinforcement phase contains defects to promotemigration of sodium ions.
 12. The dendrite penetration-resistant layerof claim 1 wherein the sodium-containing species contains at least twospecies selected from Na₂CO₃, Na₂O, Na₂C₂O₄, NaOH, NaX, ROCO₂Na, HCONa,RONa, (ROCO₂Na)₂, (CH₂OCO₂Na)₂, Na₂S, Na_(x)SO_(y), or combinationsthereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and1≤y≤4.
 13. A process for producing the dendrite penetration-resistantlayer of claim 1, said process comprising (a) preparing a workingelectrode containing a structure of said amorphous carbon matrix andsaid carbon-containing reinforcement phase dispersed in said matrix; (b)preparing a counter electrode containing sodium metal or alloy; (c)bringing said working electrode and said counter electrode in contactwith an electrolyte containing a solvent and a sodium salt dissolved insaid solvent; and (d) applying a current or voltage to said workingelectrode and said counter electrode to induce an electrochemicaloxidative decomposition and/or a reductive decomposition of saidelectrolyte and/or said salt for forming said sodium-containing speciesto produce said dendrite penetration-resistant layer.
 14. The process ofclaim 13, wherein the sodium salt is selected from sodium perchlorate,NaClO₄, sodium hexafluorophosphate, NaPF₆, sodium borofluoride, NaBF₄,sodium hexafluoroarsenide, sodium trifluoro-metasulfonate, NaCF₃SO₃,bis-trifluoromethyl sulfonylimide sodium, NaN(CF₃SO₂)₂, sodiumtrifluoromethanesulfonimide, NaTFSI, bis-trifluoromethyl sulfonylimidesodium, NaN(CF₃SO₂)₂, and combinations thereof.
 15. The process of claim13, wherein the solvent is selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, an ionic liquidsolvent, and combinations thereof.
 16. The process of claim 13, which isa roll-to-roll process that includes preparing the working electrode ina roll form supported by a feeder roller, and said step of bringing saidworking electrode and said counter electrode in contact with saidelectrolyte contains unwinding said working electrode from said feederroller, and feeding said working electrode into said electrolyte.
 17. Aprocess for producing the dendrite penetration-resistant layer of claim1, the process comprising (a) preparing a working electrode containing aporous structure of the amorphous carbon matrix and saidcarbon-containing reinforcement phase; (b) preparing a counter electrodecontaining sodium metal or alloy; and (c) bringing said workingelectrode and said counter electrode in physical contact with each otherand in contact with an electrolyte containing a solvent and a sodiumsalt dissolved in said solvent; wherein said working electrode and saidcounter electrode are brought to be at the same electrochemicalpotential level, inducing a chemical reaction between said sodium metalor alloy and said amorphous carbon matrix and inducing electrochemicaldecomposition of said electrolyte for forming said sodium-containingspecies to produce said dendrite penetration-resistant layer eitheroutside of or inside an intended rechargeable alkali metal battery. 18.The process of claim 17, which is conducted in a roll-to-roll manneroutside of said intended rechargeable alkali metal battery.
 19. Aprocess for producing the dendrite penetration-resistant layer of claim1, the process comprising (a) preparing an alkali metal battery cellcomprising an anode alkali metal layer, a layer of amorphous carbonmatrix containing said carbon-containing reinforcement phase dispersedtherein, a porous separator layer and electrolyte, and a cathode layer,wherein said layer of amorphous carbon matrix is laminated between saidalkali metal layer and said porous separator layer, and said porousseparator layer is disposed between said layer of amorphous carbonmatrix and said cathode layer; and (b) subjecting said battery cell to avoltage/current treatment that induces electrochemical reductive and/oroxidative decomposition of said electrolyte to form saidsodium-containing species to form the dendrite penetration-resistantlayer in said battery cell.
 20. The process of claim 19, wherein thestep (a) of preparing an alkali metal battery cell comprises depositingsaid amorphous carbon matrix and said carbon-containing reinforcementphase onto said alkali metal layer to form a layer having a thicknessfrom 10 nm to 20 μm.